Fix 11th, 12th, 13th and added unit tests

I had previously written this function, and noticed you had a few issues with things like:
11: 11st
12: 12nd
13: 13rd

I also added some unit tests using the QUnit framework.
Lastly, my refactor includes some performance improvements. See jsperf test here:
http://jsperf.com/nth-js-refactor-efficiency