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
four space indentation and assertGreater for __gt__ coverage
Test convert on import
Use new MediaFile api to embed art
catch ReadError when calling item.read()
write: fix undefined reference to MediaFile
revise docs for expanded RG plugin

Among other things, this clarifies which options are available with each
backend.
rename Item.media_fields to _media_fields

Following the convention of the other field sets and such. This helps avoid
any confusion with user-specified fields (although it's unlikely people will
want to name a flexible field "media_fields" :).
MediaFile.update() no longer writes files

You now call update() followed by save(). This is mainly because the
implementation no longer performs lazy updates, so the need to pair tag
setting and writing together was diminished. Benefits in cleanliness:
- No need to duplicate the id3v2 option.
- Exception handling can be more precise.
- More closely resembles dict.update().
fix Sphinx syntax in docstrings
Merge pull request #642 from ktkonrad/enum34

use the new python enum instead of hand-rolled version
changelog note for #642
remove unnecessary IndexableEnum

This is not really critical since list(Type)[i] works just as well. This fixes
a couple other problems from the enum transition:
- The name of TYPES was not really consistent with its new role as an enum
  type. Renamed this ImageType.
- Fix a rather egregious bug in embedart (undefined reference to Image).
fold OrderedEnum into autotag module

This puts the OrderedEnum generic class next to where it is actually used. It
also refers to the recipe it is taken from on docs.python.org. I also took the
opportunity to give this a capitalized name (since it's a proper type).
Remove tox's system site packages
type_index for mediafile images defaults to "other"

The `Image.type_index` property is only used for tags that require an integer
to store the cover type like ID3v2. If the type of an image is not set we let
`type_index` return the integer for an "other" image. Other than a default
"front" value, this makes sure we do not inadvertidly add an image that will
be displayed in a player.
Flake fixes
More flake fixes