Add isDeleted method to RLMObject

@tgoyne @jpsim 

Fixes issues in #803 
I'm torn. I agree this would be less likely to conflict with user property names, but I don't think it is more consistent with our other naming - on RLMArray there is a property called isReadOnly which is consistent with this, and the other methods which begin with objects[s] are all query methods which aren't really related.