Log release ID for invalid MBIDs
use the new python enum instead of hand-rolled version
add enum34 to install_requirments
add util test file with tests for enumerations. fix super() bug
Add MediaFile.fields() method
Add MediaFile.update() method to supersede save()
Test that we can migrate ITEM_KEYS_WRITABLE

The test show that we can replace the hard-coded `ITEM_KEYS_WRITABLE` constant
with the computed value derived from `MediaField.fields()` and ITEM_KEYS. This
will be done in the next commit.
Compute ITEM_KEYS_WRITABLE

See 65d64a7d50612df4ee6127ff05e591ae6fcceac0 why this is possible.
Add LazySave tests for MediaFile
MediaFile can be extended with custom fields
Use indexed format with python26
Use readable_fields() to replace ITEM_KEYS_META
Migrate ITEM_KEYS_META
Read custom fields into database
four space indentation and assertGreater for __gt__ coverage
Add Item.media_fields

This new property controls which fields to read from a media file.
Remove ITEM_KEYS_WRITABLE
Plugins can now extend MediaField
Remove lazy update from MediaFile
replaygain: modified current implementation to fit a backend framework in anticipation of other implementations