beets: Allow to configure plugin dependencies.

This also fleshes out/fixes the unit tests, which I've used for
gathering the individual requirements.

Along various Python dependencies we now also have a build-time
dependency on bashInteractive and a runtime dependency on
bashCompletion, which is needed for command line completion to work
correctly.

However, some tests for the shell completion fail at the moment, so I've
disabled them for now.

The patch for fixing mediafile codec info is a modified version of
sampsyo/beets@903e88a, where I just dropped the second hunk modifying
the changelog. It is already merged to master and thus expected to be in
the next upstream version.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>