Rewrote ID3Manager, replaced EyeD3 with Mutagen.

ID3Manager is now called MetadataManager. Getter and setter methods with
immediate persistence have been replaced with properties and a separate
save() method.

Integration into indexer and models is not yet done and will follow with
the next commit.
shiva-indexer with docopt

Conflicts:
	indexer.py
	setup.py
Added docopt to requirements.pip
Rewrote indexer to use new MetadataManager
Better verbose logging
Convert audio length to an int
Replaced ACCEPTED_FORMATS with VALID_FILE_EXTENSIONS in indexer (refs #30)
Renamed count to track_count in indexer.py
Date parsing using dateutil
Improved argument handling in indexer
Return None as default year, not empty string
Added --reindex option to indexer
Fixed issue with release year parsing