Fix: Individual column sorting was not distinct for multiple tables

- The `settings.aoPreSearchCols` array was being shared between every
  table on the table (test using a simple:
    $.fn.dataTableSettings[0].aoColumns ===
    $.fn.dataTableSettings[1].aoColumns
  ) which meant that a column filter aplpied to one table would be
  applied to all others.

- Fix is to deep copy the search model when adding a columns

- Fixes DataTables/DataTables issue #213