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-rxjava/pom.xml

@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+		 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+		 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+
+	<artifactId>spring-cloud-stream-rxjava</artifactId>
+	<packaging>jar</packaging>
+	<name>spring-cloud-stream-rxjava</name>
+	<description>RxJava support for spring cloud stream modules</description>
+
+	<parent>
+		<groupId>org.springframework.cloud</groupId>
+		<artifactId>spring-cloud-stream-parent</artifactId>
+		<version>1.0.0.BUILD-SNAPSHOT</version>
+	</parent>
+
+	<dependencies>
+		<dependency>
+			<groupId>org.springframework.cloud</groupId>
+			<artifactId>spring-cloud-stream</artifactId>
+		</dependency>
+		<dependency>
+			<groupId>org.springframework.integration</groupId>
+			<artifactId>spring-integration-core</artifactId>
+		</dependency>
+		<dependency>
+			<groupId>io.reactivex</groupId>
+			<artifactId>rxjava</artifactId>
+		</dependency>
+	</dependencies>
+</project>