Support flexible bound element types

Fixes #519

Introduces some internal changes to the framework allowing the use
of other types than MessageChannel/SubscribableChannel as bindable
types (e.g. Flux, Observable, KStream, etc.)

- Modify BinderFactory to allow the retrieval and lookup of a
  binder not only by name, but also by binding target type
- Subsequent changes to ChannelBindingService and tests to account
  for the modified signature
- Introduce BindingTargetFactory as the contract for creating bound
  elements
- Remove any references to chanels and bound elements and use
  'binding target' systematically across the board

Reinstate our own Checkstyle checks with a reduced set of rules so that header
  validation can be performed automatically at compile time.

Use ${project.version} for the checkstyle plugin configuration

Renaming some occurences of 'boundElement' to 'bindingTarget'

Renamed a stray occurence of 'channel'

Fix some occurences of String concatenation in the same line after reformatting

spring-cloud-stream-test-support/src/main/java/org/springframework/cloud/stream/test/binder/TestSupportBinderAutoConfiguration.java

@@ -19,6 +19,8 @@ package org.springframework.cloud.stream.test.binder;
 import org.springframework.boot.autoconfigure.AutoConfigureOrder;
 import org.springframework.cloud.stream.binder.Binder;
 import org.springframework.cloud.stream.binder.BinderFactory;
+import org.springframework.cloud.stream.binder.ConsumerProperties;
+import org.springframework.cloud.stream.binder.ProducerProperties;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.Ordered;
@@ -45,15 +47,15 @@ public class TestSupportBinderAutoConfiguration {
 	public BinderFactory binderFactory() {
 		return new BinderFactory() {
 			@Override
-			public Binder getBinder(String configurationName) {
-				return messageChannelBinder;
+			public <T> Binder<T, ? extends ConsumerProperties, ? extends ProducerProperties> getBinder(String configurationName, Class<? extends T> bindableType) {
+				return (Binder<T, ? extends ConsumerProperties, ? extends ProducerProperties>) messageChannelBinder;
 			}
 		};
 	}
 
 	@Bean
-	public MessageCollector messageCollector(BinderFactory<MessageChannel> binderFactory) {
-		return ((TestSupportBinder) binderFactory.getBinder(null)).messageCollector();
+	public MessageCollector messageCollector(BinderFactory binderFactory) {
+		return ((TestSupportBinder) binderFactory.getBinder(null, MessageChannel.class)).messageCollector();
 	}
 
 }

spring-cloud-stream-test-support/src/test/java/org/springframework/cloud/stream/test/aggregate/AggregateTestWithBean.java

@@ -1,3 +1,19 @@
+/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 package org.springframework.cloud.stream.test.aggregate;
 
 import java.util.concurrent.TimeUnit;

spring-cloud-stream-test-support/src/test/java/org/springframework/cloud/stream/test/aggregate/AggregateTestWithMain.java

@@ -1,3 +1,19 @@
+/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 package org.springframework.cloud.stream.test.aggregate;
 
 import java.util.concurrent.TimeUnit;

spring-cloud-stream-test-support/src/test/java/org/springframework/cloud/stream/test/example/ExampleTest.java

@@ -29,7 +29,6 @@ import org.springframework.cloud.stream.messaging.Processor;
 import org.springframework.cloud.stream.test.binder.MessageCollector;
 import org.springframework.integration.annotation.Transformer;
 import org.springframework.messaging.Message;
-import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.support.GenericMessage;
 import org.springframework.test.annotation.DirtiesContext;
 import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
@@ -46,7 +45,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 public class ExampleTest {
 
 	@Autowired
-	private BinderFactory<MessageChannel> binderFactory;
+	private BinderFactory binderFactory;
 
 	@Autowired
 	private MessageCollector messageCollector;