Add support for transcoding into the Opus format

FEATURES:
* Added transcoding support for Opus codec if ffmpeg is compiled with
support for the libopus library, patch by Martin Brodbeck.

Thanks, Martin! Works fine with ffmpeg 1.1.3 here. Can you please also
test with libav's ffmpeg wrapper?

REVIEW: 108907
DIGEST: Amarok now supports transcoding into Opus format if ffmpeg supports it
CCMAIL: Martin Brodbeck