Make the check for inheriting from RLMObject based only on the name

This "fixes" #690 by making it so that the schema also contains objects which inherit from a different RLMObject. The given sample works and all tests pass, but I'm still not at all confident that this is actually a safe thing to be doing. Any thoughts on ways this could explode on us? I also need to make sure it's not going to significantly hurt startup performance.

@alazier