High-level SkyCoordinate class (WIP)

This is a very early look at the coordinates high level interface and the `SkyCoordinate` class.

What it does right now is pass most of the API specification (within the constraint of using the legacy coordinates classes).  Specifically it can run https://gist.github.com/taldcroft/11093079 without crashing.

The hope now is to get early comments on design, and potentially contributions (as PR's on my branch) along the lines of:
- Formal tests, starting with coding up what's in the API spec, with xfails for stuff that depends on the refactor or isn't implemented yet.
- Adding high-level interface functions.
- Documentation

@cadair - you were asking how you can help move things forward so we can make 0.4??