Added core functionality for thumbnails.
Refined our sorl thumbnail config a bit.

If you're using the dev settings you can simply use your existing DB
as your sorl thumbnail key-value store. If you're in prod, you'll need
to supply a Redis service.

Note! sorl-thumbnail is a new app. You should run syncdb.

Fixes #510
Merge branch 'develop' of github.com:harvard-lil/perma into develop