av_read_frame was not yet exported to cffi.

--HG--
branch : pure-cffi
update demuxer tests according to the last spec.

And the official spec should be updated too.

--HG--
branch : pure-cffi
API updates

ready flag and proper ProcessingError
merge read_pkt into read_frame
sketch out FFPacket, but it has to be merged into Packet.

--HG--
branch : pure-cffi
Merge ../pyrana