Add closest pair of points.

Implements http://rosettacode.org/wiki/Closest-pair_problem using the [plane sweep algorithm](http://www.cs.mcgill.ca/~cs251/ClosestPair/ClosestPairPS.html) (aka the sweepline algorithm) for the closest pair problem.
Merge pull request #258 from TimDumol/closest-pair

Add closest pair of points.