Add a RACObserveChanges which only fires if the value actually changes

The current implementation of RACObserve is a little misleading. If the setter for a property is invoked with the same value as is currently present, it will actually still send a next with the "new" value, which is actually just the current value. So I added a new RACObserveChanges which does exactly this. It filters and only sends a next if the value has actually changed.