Add ntpath to include list for py2app

in the hope (since I don;t have a Mac and cannot test this) that this
fixes the error reported in
(PICARD-607)[http://tickets.musicbrainz.org/browse/PICARD-607].
Merge pull request #330 from Sophist-UK/sophist_fix_PICARD_607

Add ntpath to include list for py2app ...