Add isDeleted method to RLMObject

@tgoyne @jpsim 

Fixes issues in #803 
We could call is ```isDeletedFromRealm```? The property would be called ```deletedFromRealm``` the same way we do here: https://github.com/realm/realm-cocoa/blob/master/Realm/RLMArray.h#L62

Would this be an improvement?