"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
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.
