ffmpeg: add libopus support

Signed-off-by: Ian Leonard <antonlacon@gmail.com>
ffmpeg: Update to 2.3.4

Signed-off-by: Ian Leonard <antonlacon@gmail.com>
Merge pull request #421 from antonlacon/ffmpeg-2.3.x

ffmpeg: update to 2.3.4 and add libopus support to libffmpeg