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 12/16/2014 04:44 PM, Austin wrote:
> It is a subclass. It may not be necessary since I check that status
> code, but I thought I would be overly complete for now and scale back.
> What do you think @rbarlow <https://github.com/rbarlow>?

Ah, if it's a subclass it's fine to keep this assertion if you like!