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?
If you're not specifying any new names, you don't need the ``frame_specific_representation_info`` at all.  If you just start from ``default_representation``, this will work fine.