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.
On 12/18/2014 10:24 AM, Austin wrote:
> @rbarlow <https://github.com/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.

Fair enough. Proceed ☺