Refactor the test on demuxer stream to avoid duplicated code
Demuxer do not buffer. Decoder do.
require libavcodec exactly 54.x.y
add more AVCodecContext fields
export video width and height.
Another refactor of the demuxer test
add basic coverage for raw packet access.
add basic coverage for raw packet access. /2
some modifications as suggested by pep8
merge
Demuxer have no requirements about flush()

By API spec, Demuxers don't need to have a flush() method.
Moreover, is already (with a reason) implemented as
empty and raising NotImplementedError, so it has very little value.
Bottom line: drop it.
drop the Demuxer.flush unit test
add a few more docs
explain the problem lurking under the surface.
move Packet class and support functions in a separate (sub)module.
first WIP sketches of pyrana.video.Decoder.decode()/flush().
Pull from HG.