Changing the items collection breaks multisort.

The sortInfo.columns would have its length reset to 0 while adding back
in the first sort column, meaning that the 'forEach' would only process
the first column before exiting.
Made sortColumnsInit work on a local array.

The sortColumnsInit function would work on an array that's global to the
class, which would then be emptied before the sortData function consumed
the 'cols'.  This would result in an empty sort.
Merge pull request #1 from InteractiveIntelligence/inin-bug-multisort

Changing the items collection breaks multisort.