Add get_django_version to lazy_django.py
Add support for custom user modules introduced in Django 1.5

Since Django >= 1.5 the username field is variable, so
get 'username field' by using UserModel.USERNAME_FIELD
Use get_django_version to avoid duplication of code

This also avoids hacks to mute pyflakes.
Add a new test app which has a custom user model
Add test env to test the interaction with custom user models
Add test, fix error with custom username field
Merge branch 'Temp' into Issue-124
Fix typos