Update dependency to latest mutagen version

Mutagen 0.20 has a bug when querying bitrate of Musepack files. Let's make life
easier on everyone and make sure they have up to date version. As a bonus
mediafile tests don't fail with 0.21
Fix python namespaces for test runs

We need to make sure we don't use namespaced versions that are already installed
on the system but rather use local version from current sources
fix VFS tests

The configuration was not loaded for these tests because they didn't inherit
the common test harness. These failures were hidden on my system because of
some kind of dependency on another test.
Merge pull request #245 from sochotnicky/master

Update mutagen dependency to 0.21
Merge pull request #244 from jesseward/delete-metadata-mediafile

- changes to mediafile.py. Provides a delete method which allows the user to clear current metadata information...
mediafile: delete() works with ASF (#244)