build an inbound source that computes a moving average

see [this example from Spring XD](https://github.com/mbogoevici/spring-xd-demo) in the inbound `source` module and the [requisite Spring Integration and RxJava `MessageHandler` implementation](https://github.com/spring-projects/spring-xd/blob/master/spring-xd-rxjava/src/main/java/org/springframework/xd/rxjava/SubjectMessageHandler.java) 
