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?
I mean a one-sentence description where the x, y, z axis are pointing.
Like [here](http://www.astro.virginia.edu/~dln5q/research/idl/lbd2xyz.pro) ... apologies, this is what came up when I Google'd it.
> ; The X, Y and Z coordinates are with respect to the Galactic Center
;  Z is going up, towards b=90
;  X is towards the galactic center from our direction, L=0
;  Y is towards the L=90 direction

I think this info is really important ... adding one or two examples to transform from `Galactocentric` to `Galactic` like the one @astrofrog posted in the PR would be nice, but is not essential.