Add a map image background to the homepage

Adds a simple image background that should work up to about 1920px screens.
This does add quite a bit of weight to the page (~150k) so I guess it's
something to consider more carefully when we're doing the design for real.

Closes #4
I've copied the way the NSW theme does this, which by adding `//=require screen` to the theme's css in `application.css.scss` above allows you to get all the original styles and then override them.