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.

"origin" is an optional field that stores the URI for the site
that the image has been saved from, it's going to be used only from
bookmarklet. Fixes #63

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

As pointed in issue #75 we should get away with just checking if the pin
submitter is the currently logged in user. Assuming that we can implement
authorization for updating and deleting pins rather easily by subclassing
DjangoAuthorization so it passes the object to the Authorization backend.

This is going to be used by a "similar images" feature #74

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

It didn't make sense to call it upload-pin as it is being used only for
creating the Image object.

Fixes #81