STOMP protocol codec

The STOMP codec based on @sskachkov's initial work.  My modifications are basically class renames and miscellaneous clean-up.

Once this is merged, I'm going to move on to the following tasks that are directly relevant to this pull request:

* Remove the duplication between HttpHeaders, SpdyHeaders, and StompHeaders.
* Remove the duplication between HttpObjectAggregator, StompSubframeAggregator, etc.