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.
@nmittler - I have the client examples consolidated into the original example and this has made an issue visible that I would like your feedback on.  `InboundHttp2ToHttpAdapter` is now subscribing to the `Http2Connection.Listener` interface get the to get the `streamRemoved` events which are used to clean up the internal map of stream id to message in all (particularly exception) cases.  The ordering of events in the `AbstractHttp2ConnectionHandler$FrameReadObserver` is to call `closeRemoteSide` then dispatch the header/data event ([here](https://github.com/netty/netty/blob/master/codec-http2/src/main/java/io/netty/handler/codec/http2/AbstractHttp2ConnectionHandler.java#L785) in `onDataRead` and [here](https://github.com/netty/netty/blob/master/codec-http2/src/main/java/io/netty/handler/codec/http2/AbstractHttp2ConnectionHandler.java#L864) in `onHeadersRead`).  This is causing `InboundHttp2ToHttpAdapter` to clean up the stream before the `onDataRead` is called and results in an unknown stream event.  I don't necessarily want to change the order of events because I see why cleaning up socket resources asap is advantageous (and is also symmetric with the `closeLocalSide` logic).  How would you recommend I proceed?