fix #37 - send email when askers' bids are met

This time using django-mailer, but I'm not sold on it. Could just as easily use built-in django mail if we think that's just as good?
I don't think I follow your reasoning against the model_mommy approach? The 2 articles you cited in the comment both exempt model-level test code from isolated testing.

* "Many of those mock-free tests are on models, which I test in integration with the database." -[Destroy All Software](https://www.destroyallsoftware.com/blog/2014/test-isolation-is-about-avoiding-mocks)
* "At the models layer, we no longer need to write isolated tests—the whole point of the models layer is to integrate with the database, so it’s appropriate to write integrated tests" -[TDD with Python](http://chimera.labs.oreilly.com/books/1234000000754/ch19.html)