Content - ContentTypes ContentTypeResource and ContentUnitsCollections

I thought these three would share more code than they ended up sharing so I combined them. The next one looks more complex though, so decided to make this PR now.
@rbarlow, I think it wouldn't be a good use of time to modify `run-tests.py` if we are probably going to be using the django test runner soon. In the meantime it seems that the simplest and least sloppy solution is simply to set the environment variable before running nosetests.