First cut of frame encoding/decoding and session management for HTTP2

Motivation:

Needed a rough performance comparison between SPDY and HTTP 2.0 framing.
Expected performance gains were seen in HTTP 2.0 due to header
compression.

Modifications:

Added a new codec-http2 module containing all of the new source and unit
tests.  Updated the top-level pom.xml to add this as a child module.

Result:

Netty will have basic support for HTTP2.