New Http2HttpDecoder to tranlate HTTP/2 frame read events into HttObjects

Netty Version: 5.0.0.Alpha2-SNAPSHOT

Context: To address #2658.

**Details**
The focus here is the `Http2HttpDecoder` class.  There are additional classes (i.e. `DelegatingHttp2HttpConnectionHandler`) to support common operations that have not yet made it into the HTTP/2 codeline.  It may be worth pulling these changes out into another pull-request and re-factoring the examples.

There is a new example to demonstrate the new functionality.  The focus here is on being able to support HttpObject operations such as `HttpContentDecompressor`.

@normanmaurer and @nmittler thanks for the suggestions and feel free to review and provide feedback.  There are a few TODO comments that I would particularly be interested in feedback on.
Unless @normanmaurer has objections I can change to InboundHttp2ToHttpAdapter.  I don't think this is too wordy (I would rather it be concise to its functionality).  I also agree with your points related to this class not providing the typical "decoder" interface.