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?
@eteq Yes, I like that description better, but it actually means I need to update the code (to, I think, a better parametrization). Right now, the final roll angle is set to be the angle needed to align the XZ plane from its final configuration to Galactic XZ, but you are suggesting that we define the parameter for the class to be a deviation from this pre-set angle, right?