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 don't see this travis-ci test fail locally:
https://travis-ci.org/astropy/astropy/jobs/44502838#L2590

But I think if you explicity put
```
diff = np.array(np.abs(g_xyz - gc_xyz))
```
you are guaranteed that `diff` will be a `np.ndarray` and not a `Quantity` and these asserts should always work.