Retain unknown frames for possible upstream handling.

Rather than extend the lower decoders to handle HTTP/2 frame parsing extensions, it seems reasonable to instead not process the frame and mark it as an UknownFrame.  This PR switchs the fhType of the FrameHeader to retain the frame type id. This way a stream/connection handler can do additional frame processing on unknown frames if its configured to do so, or ignore them.

Fixes #1.