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?
Right, exactly.  I was thinking that's what it was, but now I see you're right that it's not!  I guess you can just put something like  ``ROLL0 = 148.5986320*u.degree``  at the top of the module and just replace all your current ``galactocentric_coord.roll`` with ``galactocentric_coord.rol - ROLL0``. (and change the default to 0, of course)