Implement galactocentric frame

This is a first stab at implementing a frame for galactocentric coordinates. Some open questions in my mind: 

* Do we want to allow a transformation to a left-handed system as well? (because some people are crazy)
* Can we think of better names for the frame attributes?
Random inputs to tests make for random failures later (e.g. if there are numeric problems near the poles or something).
Instead a utility function should be added that generates deterministic grids of positions on the sky and used as input for all the coordinate tests.