Support subclassing RLMObjects

Does not support polymorphic behavior.

@tgoyne 
I agree with you it would be nice to improve this. Another thing to consider is for the dynamic interface where we may have vanilla RLMObjects, so I don't think class comparison will work. The only non-string comparison I can think of is comparing table indexes.

Another option is to use uniqued strings for classnames, so that we can compare classname strings using pointer equality. This would require a lot of code change but could be worth it if this is something we have to do all over the place.