add documentation

[petebachant]

If one uses a single point, e.g. a 3-tuple as input, this will fail, but one could easily put this inside the function before checking for the proper shape:

points = np.asarray(points, dtype=np.float32)

[chichilalescu]

as per the discussion for #14, basic functionality should be well documented. i.e. add the stuff that sphinx api-doc would like to see.