INT-3147 Twitter/Feed expose MessageSources as beans

* In the Twitter Inbound Channel Adapter Parser, expose the MessageSource as top-level bean rather than as inner bean
* In the Feed Inbound Channel Adapter Parser, expose the MessageSource as top-level bean rather than as inner bean
* Add tests
* Add documentation to Twitter + Feed chapter
    
Jira: https://jira.springsource.org/browse/INT-3147