Use QThreadPool for threading
Use separate thread pool for file saving

With a thread count of 1, to avoid race conditions in File._save_and_rename.
Merge branch 'master' into new-threadpool

Conflicts:
	picard/ui/mainwindow.py
Merge pull request #146 from mwiencek/new-threadpool

Use QThreadPool for threading
Remove a superfluous import of picard.util.queue

It has been removed, so this import made picard unstartable on clean
checkout (old ones most likely has a .pyc file of the queue module lying
around).
Allow tagger script to access file metadata

Fixes PICARD-259 (and associated sub-tickets)

This moves the taggerscript evaluation to happen when a file is matched
(i.e. when a file is added to a track), where it has access to the both
the file metadata *and* the track metadata.

Because the track metadata no longer has the post-taggerscript data, the
preview of an unmatched track in the metadata box would be incorrect, so
I evalute the taggerscript there (on a copy of the data...) to handle
the special case.