add string month year date sort

This is a sort plugin to compare dates like "January 2015" (string format MMMM yyyy). I've seen a [couple](http://stackoverflow.com/questions/19769121/datatables-date-sorting-not-working-on-firefox-ie) [people](http://datatables.net/forums/discussion/1242/sorting-dates-with-only-month-and-year) looking to do this (myself included), so I took some code I found in the forums and updated it into a plugin.