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?
I think you should pick some reasonably sensible default like 8 kpc here.  If you use None, I get not terribly useful errors about not being able to divide by `NoneType`.