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?
Ah, yes - I tried to do this in upper part of the docstring:

    The position of the Sun is assumed to be on the x axis of the final, right-handed system.

but I agree that I can make this more clear.