Add compat.py to get get_user_model working
Getting user model directly from settings
Get user model from compact instead of django directly

To prevent:
```
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.
```
Code cleanup on tests module
Code cleanup on fields module
Okay, srsly fixing getting model error
Running test suite with travis on multiple Django versions.
Update changelog for getting user model error
Update changelog
Merge pull request #12 from Alir3z4/fix-#11-import-fails-django-17

Fix #11 import fails django 17
Fixes #11