Add night sky brightness calculator

This is similar to #27 by @ejeschke, but I'm making a new issue because what I propose here is a larger and probably post-GSoC project.

What really matters for observing is the night-sky brightness in the target direction, not just the moon phase or separation to the moon.

There's models for the contribution from scattered moonlight:
- http://labs.adsabs.harvard.edu/adsabsadsabs/abs/1991PASP..103.1033K/
- http://labs.adsabs.harvard.edu/adsabsadsabs/abs/2001MNRAS.322..201B/
- http://www.eso.org/sci/facilities/eelt/science/drm/tech_data/background/

It would be great to have some night sky brightness calculator implemented in astroplan.
I don't know if there's a really simple model that can be implemented in a day, or if they are all complex.

Start with issue #27 and the three cited scattered-moonlight models; no source file, test, or entry point is named here, so first determine which model and calculator scope astroplan should support. Done means an agreed night-sky-brightness calculation for a target direction is implemented and validated, with tests covering the chosen model.

Written by the indexing model from the issue text.