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.
The refactor can now be used in an AMD environment (require.js).
Lastly, my refactor includes some performance improvements. See jsperf test here:
http://jsperf.com/nth-js-refactor-efficiency