ENH: add distance gufuncs to `scipy.spatial`

This provides a new submodule `umath_distance` to `scipy.spatial`, that provides gufuncs for distance calculations. The Python wrapper to the C coded gufuncs allows `pdist` and `cdist` style calculations with a simpler, common interface.

Only functions of two inputs with no additional parameters are included in this version, i.e. everything provided in `scipy.spatial.distance` except `mahalanobis`, `minkowski`, `seuclidean` and `wminkowski`.