INT-2867: Add Script Support to inbound-c-a

* Add `<script>` and `<expression>` to `<int:inbound-channel-adapter>`
* Refactoring for `DefaultInboundChannelAdapterParser`
* Introduce `ScriptExecutingMessageSource` to allow scripts' `MessageProcessor`
implementations work as a source for polling endpoint

JIRA: https://jira.springsource.org/browse/INT-2867

INT-2867: documentation

In addition the tests for Lifecycle attributes

Polishing

spring-integration-scripting/src/main/java/org/springframework/integration/scripting/ScriptExecutingMessageSource.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2013 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.integration.scripting;
+
+import org.springframework.integration.endpoint.AbstractMessageSource;
+
+/**
+ * The {@link org.springframework.integration.core.MessageSource} strategy implementation
+ * to produce a {@link org.springframework.integration.Message} from underlying
+ * {@linkplain #scriptMessageProcessor} for polling endpoints.
+ *
+ * @author Artem Bilan
+ * @since 3.0
+ */
+class ScriptExecutingMessageSource extends AbstractMessageSource<Object> {
+
+	private final AbstractScriptExecutingMessageProcessor<?> scriptMessageProcessor;
+
+	public ScriptExecutingMessageSource(AbstractScriptExecutingMessageProcessor<?> scriptMessageProcessor) {
+		this.scriptMessageProcessor = scriptMessageProcessor;
+	}
+
+	@Override
+	protected Object doReceive() {
+		return scriptMessageProcessor.processMessage(null);
+	}
+}