Overall refactoring of the STOMP codec

- StompObject -> StompSubframe
- StompFrame -> StompHeadersSubframe
- StompContent -> StompContntSubframe
- FullStompFrame -> StompFrame
- StompEncoder/Decoder -> StompSubframeEncoder/Decoder
- StompAggregator -> StompSubframeAggregator
- Simplify the example
- Update Javadoc
- Miscellaneous cleanup