HTTP/2.0 draft 14 update (frames)

Resolves (at least partly) issue #1015.

Update the HTTP 2.0 Variant to support HTTP/2.0 draft 14, in which the [frame layout changed](http://tools.ietf.org/html/draft-ietf-httpbis-http2-14#section-4.1) from the previous draft.

One thing I'm confused by is that the new frame header is supposed to be 9 octets. I can see where 8 of them (type, flags, padding, reserved bit, stream identifier) are, but I can't figure out where the 9th one is.
is the full 2^24 available? Just want to make sure we get the sign bit correct