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. 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.

Upgraded to hpack 0.7.0, which is used by draft12. Also removed
draft10 code and moved draft12 code to the ../http2 package
(no draft subpackage).

Result:

Addition of a HTTP/2 draft 12 support.