Planes can update from buffers

Attempt to make frames from Numpy arrays more efficient. (#46)
Frame.pts always comes from AVFrame.pkt_pts when decoding

Dranger's tutorials appear to be dangerously out of date in this respect.
Centralize copying of frame attributes
Move seek flushes to the container, and fix decode flushes
Python 2.6 does not have memory views
Expose all seeking flags
Make sure we have a codec before decoding
Skip subtitle tests if we don't have the codec

Libav does not have the one for movtext.