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.
+1

> Am 26.07.2014 um 21:49 schrieb Nathan Mittler <notifications@github.com>:
> 
> In example/src/main/java/io/netty/example/http2/client/httpobject/Http2Client.java:
> 
> > +import io.netty.util.concurrent.GenericFutureListener;
> > +
> > +import java.util.Arrays;
> > +import java.util.concurrent.ScheduledExecutorService;
> > +import java.util.concurrent.ScheduledFuture;
> > +import java.util.concurrent.TimeUnit;
> > +import java.util.concurrent.TimeoutException;
> > +
> > +import static io.netty.handler.codec.http.HttpMethod.*;
> > +import static io.netty.handler.codec.http.HttpVersion.*;
> > +
> > +/**
> > + * Provides a client that waits (and provides asynchronous notification) for the connection and Http2Settings object to
> > + * be read
> > + **/
> > +public final class Http2Client implements Http2EventListener<Http2Settings> {
> Should we just update the existing client/server example to use the new conversion to HTTP? Seems like that would allow us to demonstrate all the goodies at once :)
> 
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