HTTP Content Encoder allow EmptyLastHttpContent

Motiviation:
The HttpContentEncoder does not account for a EmptyLastHttpContent being provided as input.  This is useful in situations where the client is unable to determine if the current content chunk is the last content chunk (i.e. a proxy forwarding content when transfer encoding is chunked).

Modifications:
- HttpContentEncoder should not attempt to compress empty HttpContent objects

Result:
HttpContentEncoder supports a EmptyLastHttpContent to terminate the response.
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.