Dupe data bug on dblclick fixed

Fixed duplicate data bug caused when double clicking on a <thead> element that is sortable.  This was a race condition and caused two or more datasets to return if loading was incomplete.

* Added two new states - isLoading (boolean), and lastLoad (object | $.ajax request)
* Added state check function to abort lastLoad
* Rebuilt getJSON as $.ajax request - in order to make abort() and stop() methods accessible