Add support for HTTP/2 draft 12.

Motivation:

Draft 12 has just arrived and has quite a few changes. Need to update in
order to keep current with the spec.

Modifications:

This is a rewrite of the original (draft 10) code. There are only 2
handlers now: preface and connection. I've merged the preface
reader/writer into the preface handler since the draft 12 spec indicates
that both endpoints must send the preface. The connection handler is now
callback based rather than frame based (there are no frame classes
anymore). AbstractHttp2ConnectionHandler is the base class for any
HTTP/2 handlers. All of the stream priority logic now resides in the
outbound flow controller, and its interface exposes methods for
adding/updating priority for streams.

All of this code has been separated into a new module since it depends
on hpack-0.7.0.

Result:

Addition of a new module codec-http2-draft12.