CS193p Assignment 2.3 - Match cards properly

Previously, when selecting 3 cards, if the first two matched and the
third did not then no match would be found.  This fixes that problem by
iterating through all cards looking for matches.
CS193p Assignment 2.8 - Complete

The mix between text and mouse usage in Xcode is…disconcerting.
Clarify comment about intent of card iteration
CS193p Assignment 2.XC1 - Complete

This is more than the dozen lines of code that the assignment said was
needed, but I’m pretty certain I’ve covered more edge cases and done a
better job of maintaining the MVC division of responsibilities.