Support RxJava module

- Add a way to enable `RxJava` module
   - Add `EnableRxJavaModule` annotation that inherits `EnableModule` for `Processor` type.
      - It also registers the custom `MessageHandler` that sets up RxJava `Observable/Subscriber` to process the incoming message reactively.
      - Introduce `RxJavaProcessor` that is expected to be implemented by the module author with the input/output being `Observable`. This processor is autowired into the message handler registered above.
 - Add moving average sample

Add project `spring-cloud-stream-rxjava`

 - This project extends `spring-cloud-stream` so that any RxJava processing module would have this as a dependency

spring-cloud-stream-samples/rxjava-processor/src/test/java/demo/ModuleApplicationTests.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2015 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 demo;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.web.WebAppConfiguration;
+import org.springframework.boot.test.SpringApplicationConfiguration;
+import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
+
+@RunWith(SpringJUnit4ClassRunner.class)
+@SpringApplicationConfiguration(classes = RxJavaApplication.class)
+@WebAppConfiguration
+@DirtiesContext
+public class ModuleApplicationTests {
+
+	@Test
+	public void contextLoads() {
+	}
+
+}