Add an unknown frame payload type with the raw data.
Switch fhType in the header to the raw word8 frame type id to retain it for upstream unknown frame handling.
Merge branch 'frame' of https://github.com/kazu-yamamoto/http2 into frame-unknown

Conflicts:
	Network/HTTP2/Decode.hs
	Network/HTTP2/Encode.hs
	Network/HTTP2/Types.hs
	test/HTTP2/FrameSpec.hs
cleaning up unknownFrame.