HTTP/2 Data Compressor

Motivation:
The HTTP/2 codec currently does not provide an interface to compress data. There is an analogous case to this in the HTTP codec and it is expected to be used commonly enough that it will be beneficial to have the feature in the http2-codec.

Modifications:
- Add a class which extends DefaultHttp2ConnectionEncoder and provides hooks to an EmbeddedChannel
- Add a compressor element to the Http2Stream interface
- Update unit tests to utilize the new feature

Result:
HTTP/2 codec supports data compression.
@trustin - I think these got moved during the header back port.  I think it is better to keep a common definition in the HttpHeaderValue class rather than an independent definition in http/2 and http codec (and potentially spdy).  If you agree I will back port the portion related to this change after this PR is resolved.