Allow one to modify default behavior when moving files using a plugin

Basically, it adds a plugin hook to replace `shutil.move()` call by whatever the user prefers.
A sample (and simple) plugin was added to `contrib/plugins`

Not ready to be merged, for testing and feedback only.

Related to:
http://tickets.musicbrainz.org/browse/PICARD-623
http://tickets.musicbrainz.org/browse/PICARD-422


This is a bit fragile: after one plugin deletes `old_file`, all subsequently called functions will fail. Do we need this to be extensible via an ExtensionPoint or is the `file_move` plugin you included the only one that's going to be written in the foreseeable future?