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
four space indentation and assertGreater for __gt__ coverage
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
Make flake8 mandatory on travis

The travis build will fail if flake8 finds errors. Since not all code is
cleaned yet we ignore a couple of files in `setup.cfg`.

We'll gradualy clean the code remove the excluded files from `setup.cfg`.

See #669.
Add "shell" keyword for commands and use it in convert

Convert used to split the user command at whitespaces and pass it to `Popen`.
This approach is to naive, it also was not consistent with output from the
logs. Instead we pass the whole command to a subshell.
More robust tests for converted files
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