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.
I'll look at the layer above to see if this is handled but according to the [spec](http://tools.ietf.org/html/draft-ietf-httpbis-http2-13#section-6.4) this is a protocol_error

> RST_STREAM frames MUST be associated with a stream.  If a RST_STREAM
> frame is received with a stream identifier of 0x0, the recipient MUST
> treat this as a connection error (Section 5.4.1) of type
> PROTOCOL_ERROR.

> RST_STREAM frames MUST NOT be sent for a stream in the "idle" state.
> If a RST_STREAM frame identifying an idle stream is received, the
> recipient MUST treat this as a connection error (Section 5.4.1) of
> type PROTOCOL_ERROR.