PEP8 conformance fixes.
Remove unused dict since ac9989fd
Merge pull request #152 from zas/replace_words

Remove unused dict since ac9989fd
Merge pull request #149 from zas/PEP8_shoot_again

PEP8 conformance fixes.
Use a smarter method to extract track number from filename.

When file has no metadata field indicating the track number, Picard
is trying to guess it from filename, but the simple regex wasn't very efficient.

tracknum_from_filename() does a much better job.
A test case was added.