Add a protocol / CaptureDevice that read an ivffile that contains vp8
frames for the doRead function...

Also add a custom MediaDevice for convenience.
Use the difference between the timestamp of 2 consecutive frames read
in the IVF file to sync the stream, instead of just
using the framerate of the frame.
Fix the problem discussed here :
https://github.com/jitsi/libjitsi/pull/11#issuecomment-49404492
Also fix a bug where the first frame only was send (when loopFile was true) :

FFmpeg doesn't set the number of frame contained in the IVF file, so
the IVFFileReader was reading only the first frame because it thought
it has reached the last frame. It now use the EOF to know if the end
is reached or not.
Minor clean-up.