Now the private pin should only be viewed by owner.

Too lax unique constraints for Thumbnail coped with "on demand" thumbnail
generation may leave database in inconsistent state where two thumbnail for
the same size are saved. We should be able to prevent that from happening
by generating all thumbnails when we save the image. Should fix #24, but
I can't figure out a way to actually test it.

Removed pins django app, and moved code to the core. Moved user related
code out of core to the users app.

Rewritten register view using CBV approach, and added a authenticate/login
call after form has been validated and user created. Fixes #65

Rewritten API to handle creating pins for both urls
and previously-uploaded images. Added some tests for it.