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?
Adding an `Examples` section with one or two simple examples that let's the user quickly understand the orientation of the x, y, z axes would be great (for the system with default parameters as defined here).