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?
What do you mean by "quickly understand the orientation"? Maybe we can merge this, and you can submit a simple PR with what you were imagining?