Add dualpivotquicksort (Dual-Pivot Quicksort)

Dual-Pivot Quicksort is a variation of Quicksort by
Vladimir Yaroslavskiy.

"This algorithm offers O(n log(n)) performance on many data sets
that cause other quicksorts to degrade to quadratic performance,
and is typically faster than traditional (one-pivot) Quicksort
implementations."
-- http://download.oracle.com/javase/7/docs/api/java/util/Arrays.html
Merge pull request #17 from mhutchin/adddualpivotquicksort

Add dualpivotquicksort (Dual-Pivot Quicksort)