Revert "Remove 'mad' from 'mopidy deps' listing"

This reverts commit cb6e19c039d2a49be235a34dcf05236b3f24625a.

The "mad" element is the only way to decode MP3 with GStreamer 0.10 on OS X.
deps: Add flump3dec (and mpg123audiodec) to Gst element list
deps: Remove GStreamer mixers from element list