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.
I think I figured out what this means. I read the diagram too literally.

24-bit length, immediately followed by 8-bit type and 8-bit flags, immediately followed by stream ID, etc.
In other words, the "emptiness" is just added for readability.