Delete tags from media files
Remove all tags from empty.wma
Return None for missing tags.

Instead of returning a special "None value" if a tag does not exist, we return
none directly.
Set literal not available in py26
flake8 fixes
Delete 'None' properties from media file
Merge pull request #660 from geigerzaehler/mediafile-delete-tags

Delete and handle nonexistant tags