Support adding an offset to an S2LatLng, compute the heading/angle, etc.

Basically, I would like to see most (if not all) of the methods available in https://github.com/googlemaps/android-maps-utils/blob/master/library/src/com/google/maps/android/SphericalUtil.java to be part of an S2LatLng so that I can, for example, add X miles to an S2LatLng at a certain heading/angle.