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.

Make sure that we are using binary mode to read binary files like images, this Fixes #20

It's part of the issue #20 (weird path on Windows). Django File class used
a full file path which resulted in path like C:\Example\pinry\static\C:\... which
is obviously invalid.

Factory Boy provides us with a better way of populating complicated
fields (like M2M relations) than overriding _prepare class method,
so we should be using them.

There has been issues when running a full test suite (for pinry and
the entire Django stack), and test fixtures are hard to maintain anyway,
so rewrite tests to use a Model mocker library (Factory Boy) to populate
database.