Starting with an h264 implementation. Can be extended to support other codecs.

A few caveats:
- OpenGOP streams are currently not supported. The firt packet must be an IDR
  frame.
- In some streams, a few frames at the end may not get a reordered PTS when
  they reference frames past EOS. The code added to derive timestamps from
  previous frames needs to extended.

Addresses ticket #502.
Signed-off-by: James Almer <jamrial@gmail.com>