Started adding retina support in earnest.

Fixed the background declarations in abr-{big-,}retina.css so they display
correctly.  Took the min width down from 1024 to 966 px to allow the big
style to display on Nexus 7.
Fixed wide style to only work in portrait mode and fixed masthead spacing.

I moved the masthead up for short displays on mobile devices.
Updated readme to reflect progress on the project.