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?
There's a sphinx error that I think is coming from this.  I believe you need to remove the ``:class:`` part.

It's counter-intuitive, but I'm pretty sure that's because the code that automatically resolves things like `astropy.units.Quantity` into the real object (`astropy.units.quantity.Quantity`) isn't properly set up to recognize that this is a class and not something else.  So just referencing it without specifying the type should do the job (I hope?)