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).
Is there any reason not to do:
```python
try:
    from django.contrib.auth import get_user_model
except ImportError:
    from django.contrib.auth.models import User
else:
    User = get_user_model()
```