INT-3147 Twitter expose MessageSource as beans

* In the Twitter Inbound Channel Adapter Parser, expose the MessageSource as top-level bean rather than as inner bean
* Add tests

Jira: https://jira.springsource.org/browse/INT-3147
INT-3147 Feed expose MessageSource as bean

* In the Feed Inbound Channel Adapter Parser, expose the MessageSource as top-level bean rather than as inner bean
* Add test

Jira: https://jira.springsource.org/browse/INT-3147
INT-3147 Add documentation

* Add documentation to Twitter chapter
* Add documentation to Feed chapter