Pager: Passing jqxhr for ajaxProcessing function.

There are situations where the data of pagination (total, page, offset, last-page) are sent by headers and other situations where they are sent inside json return. The Tablesorter only offers me the second option, so I added support for the this first option. 

So I changed the code to include support for jqXHR as third option on ajaxProcessing.