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?
Remove this "TODO: reference" comment? (I think you are giving the reference 2001ApJ...553..184C in the docstring.)