Internal: Refactor sorting classes for size and efficiency

- Refactoring _fnSortingClasses to be more efficient in how it operates
  and to compress better. This is done by looping first over the
  elements to have their classes removed, and then looping over only
  those to have them added. Previous all columns were operated upon.
  Also using _pluck and that fact that all cells fromt he tbody are
  stored makes the code much smaller, more readable and compressable.
  Only 0.5KiB saved in the refactor, but every little helps...

- Restored column sorting classes functionality that had been disabled
  earlier in the 1.10 development sequence.