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 would like to use the Django test runner so for me it's a question of when and how. To do it we would need to have Jenkins switch to it, figure out how to run individual tests from the command line, run all tests from the command line, and use PyCharm to run an individual and all tests also. I plan to investigate this some in the new year and we can review what comes out of that. Tagging @barnabycourt and @mhrivnak  so they see my test runner plans for the future. :smiling_imp: 

The new Django unit tests will all inherit from unittest directly, and won't need any base test module. Usually when I can't explain some magical behavior in the testing code it's been provided by one of these base test modules. This has caused me to reach the conclusion that they do more harm than good. The Django tests don't have a need for a base test module so we aren't planning on making one. I'm hoping we can keep it that way, but let me know if I'm overlooking something.

For the immediate short term, I think moving lines 1 and 2 to near [this location in run-tests.py](https://github.com/pulp/pulp/blob/introducing-django/run-tests.py#L13) as part of a separate PR is the best thing to do. I believe @asmacdo is already doing that in a separate PR.