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?
Then it's better ... but it's still more lines of code and wasted CPU cycles.
But obviously this is not critical, we should merge this today ... I can make a PR changing this next week if @eteq or @adrn don't put it in.