We weren't checking if the Pin submitter is the logged user which made it possible
to pass any submitter to the Pin resource create call. Fix it, and make the submitter
optional.

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

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

There has been some refactoring going on in the pinry.pins.models module.
The upload_to code has been refactored into its own function, images
have been moved to their own models - otherwise the number of fields
in the Pin model would skyrocket. Also ModelManagers have been written
to move image fetching and generating outside of models.