Adding support for HTTP/2 initialization handshake.

Motivation:

The current HTTP/2 support does not properly comply with the HTTP/2 spec
wrt startup.

Modifications:

Changed the frame codec as well as the connection handler to support
exchange of the connection preface, followed immediately by an initial
settings frame.

Result:

The HTTP/2 initialization handshake will be in compliance with the spec.
Will need more work to support the upgrade protocols, however :)