Show class names in equality failure message.

This updates the failure message when comparing objects.  The same could be done elsewhere, but I'm not sure how useful it would be.

I also went for just displaying (NSString) instead of (__NCFString) or whatever class comes out of the NSString cluster.  I considered doing this for other clusters but I think that opens a can of worms (I don't know if there is a way to auto-detect the "parent class" name out of a cluster class).  There could be some confusion for the uninitiated when comparing two NSArrays that are of different classes in the NSArray cluster, for example.

Anyway, this gets the basic job done with minimal impact.  Feedback is welcome.