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staticfiles_urlpatterns() add only handler for serving /static/ path,
add pattern for /media/.

Finally had time to figure out the problem "caused" by django_compressor,
so we can no re-enable it. Fixes #3

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