Add bSortMulti option to disallow multiple-column sorting

Added a <code>bSortMulti</code> feature option. When set to <code>false</code>, pressing the shift key to sort multiple columns at once will be disabled. This is handy when simplifying the logic for dealing with server-side sorting. Based on [this forum discussion](http://datatables.net/forums/discussion/5589/possible-to-disable-sorting-of-multiple-columns#Item_2).