Move duplicated code into CodecUtil

Motivation:

We have some duplicated code that can be reused.

Modifications:

Create package private class called CodecUtil that now contains the shared code / helper method.

Result:

Less code-duplication
Changing HTTP/2 inbound flow control to use Http2FrameWriter

Motivation:

This is just some general cleanup to get rid of the FrameWriter inner
interface withing Http2InboundFlowController.  It's not necessary since
the flow controller can just use the Http2FrameWriter to send
WINDOW_UPDATE frames.

Modifications:

Updated DefaultHttp2InboundFlowController to use Http2FrameWriter.

Result:

The inbound flow control code is somewhat less smelly :).
HTTP/2 draft 14 HTTP message flow
Motivation:

HTTP/2 draft 14 came out a couple of weeks ago and we need to keep up
with the spec.

Modifications:
-Revert back to dispatching FullHttpMessage objects instead of individual HttpObjects
-Corrections to HttpObject comparitors to support test cases
-New test cases to support sending headers immediatley
-Bug fixes cleaned up to ensure the message flow is terminated properly

Result:
Netty HTTP/2 to HTTP/1.x translation layer will support the HTTP/2 draft message flow.