Centroid determination ...

This is slick, and is going to save me a fair bit of time soon.

I took a stab at the centroid functionality based upon chelm/tylr#2. The implementation seems to work, and is based on the math at http://en.wikipedia.org/wiki/Centroid#Centroid_of_polygon ...

There is an error that is thrown based upon the MultiPolygon geometry for Maryland. The first polygon has the same value for all of the y coordinates, which results in a bad calculation.