Simulate content types

This fixes #94 

We provide the option of not using the datastore at all for ContentTypes. Everything is done in memory by overriding the ContentType object manager. This has two major advantages:

 - Tests are ridiculously fast now. ContentTypes just don't agree with the datastore stub at all
 - ContentTypes only really exist for generic foreign keys, which can be done in far nicer ways
 - They don't really help on non-rel as there are no joins

This doesn't cause any new test failures and the djangae-testapp tests now run in ~40 seconds rather than ~20 minutes.