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 personally prefer fully isolated unit tests over the model_mommy approach-- see the discussion here for more https://github.com/mozilla/masterfirefoxos/pull/23#issuecomment-67976434