Added Util namespace, and started it with math_util.hpp.\nmath_util.hpp currently supplies a helper function for calculating a perpendicular 2D vector.
Improved skier physics, specifically for turning.

Skier turning should now feel more 'ski-like' (at least it does to me).  This was accomplished by using the new math_utils.hpp function  so that turning occurs in a more centripetal fassion. The system for ratios of the various physics effects in  was also modified slightly.