Log release ID for invalid MBIDs
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
Add item.try_write() to log errors

Many commands and plugins use `item.write()` to update tags. Since the success
of the call is not critical to the functionality of most consumers we want to
catch any exceptions, log an error and continue with our task. The new method
encapsulates this logic.

This fixes #675.
Flake8 fixes
Remove ITEM_* constants

We might consider renaming `Item._fields` to `Item.fields` since other parts of
beets use it as a replacement for `ITEM_KEYS`.

See also #650.
Remove ALBUM_* constants. Closes #650
Move log handler to beets.ui
embedart: catch log errors (fix #678)
remove stray ITEM_FIELDS reference (#650)
Merge pull request #676 from geigerzaehler/item-try-write

Add item.try_write() to log errors
Merge pull request #660 from geigerzaehler/mediafile-delete-tags

Delete and handle nonexistant tags
info: don't print out album art blobs
fix Sphinx complaint

Apparently, the ReST syntax doesn't like letters appearing after a
`backticked` phrase. That's sorta annoying.
Refactor zero plugin
Remove null normalization for replaygain.

A `None` value for replagain fields is different from a `0.0` value [1].

[1]: https://github.com/sampsyo/beets/issues/157#issuecomment-39521322