Remove some obsolete code

Python 2.5 is no longer supported.
Merge remote-tracking branch 'zas/info_dialog2'

Conflicts:
	picard/tagger.py
Use python-discid instead of libdiscid if available.

Python-discid (http://python-discid.readthedocs.org) is a Python binding of libdiscid.
It resolves http://tickets.musicbrainz.org/browse/PICARD-503

When python-discid isn't available, picard falls back on its internal code
to use libdiscid, at least for now.