Support Django 1.4(.13)

Currently, `incuna-mail` doesn't support Django < 1.5.  It imports `django.contrib.auth.get_user_model` which was added in 1.5, and may have other incompatibilities as well.

This is almost-blocked by #6 (and, in turn, #5).  We can fix the problem by repeatedly manually testing it, but having Travis assert that we support 1.4.x would be better, and more reliable (with decent tests).