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?
At the moment no API docs for the `Galactocentric` class are generated. You have to add this:
```
__all__ = ['Galactocentric']
```
and then check that the HTML docs look OK with `python setup.py build_sphinx`.