HTTP/2 Frame to HttpObject Adapter

Transferring @Scottmitch's personal pull request to @akamai's fork for https://github.com/netty/netty/pull/2695
On second though there is an issue here.  I am already extending `Http2ConnectionAdapter` to avoid pulling in all the `Http2Connection.Listener` methods that I don't use.  So I would have to add an additional layer in the class hierarchy to bring `Http2FrameAdapter` and `Http2ConnectionAdapter` together in order to avoid having `NOOP` methods in `InboundHttp2ToHttpAdapter`.  Is this worth it?