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


I agree it is fragile, the plugin i include is just an example, i don't think we should add one by default anyway.
I'm not very happy with this approach, but allowing one to override those relatively low level operations could help to solve specific issues without modifying Picard code.

As said: Not ready to be merged, for testing and feedback only.