Fixed several failing tests and packages

* Upgraded Django REST Framework to a newer version in requirements to be able to run the new tests from Mark (all passing).
* Removed non existent package ``django-colortools``. It doesn't exist in PyPI so, if somebody needs it, it should be installed directly from its [GitHub repo](https://github.com/neubloc/django-colortools).
* Some indentation/PEP-8 fixes around.