Add default album art for dylanaraps/bum#11

[Gadgetoid]

We looked into a couple of approaches for supplying default cover art. The simplest seems to be to leverage as much of the existing image handling/caching functionality as possible by supplying the save function with a string of bytes similar to what it would receive from the MusicBrainz API.

Thanks to @Septolum for coming up with an image that scales nicely and compresses well. It's actually a 2-colour PNG, but most image handling libraries don't care if the filename/content type mismatch. (Works for us in PIL, too)

[dylanaraps]

This is a really nice solution! 👍

[dylanaraps]

Just some very minor lint errors to fix.

[Gadgetoid]

On it!

[dylanaraps]

LGTM 👍

Thanks!