Initial implementation of parallel merge sort.

Grossly in-efficient.  Needs a much better solution for spawning tasks
to be at all comparable to standard sort.
Speed improvements to the merge-sort-par implementation.

Hardcoded limit of 8 tasks right now.  This does provide a speed up over
purely sequential implementation, but not huge yet.
Still todo:
    - Parallel merge operation
    - Pass number of threads as command line parameter