Add border-radius mixin
Add Edgecase theme

New scss, compiled css, a modified version of the “github” theme for
highlight.js, supporting fonts and imagery, and update to gruntfile.
Disable all themes except Edgecase
Tweak index.html for our purposes

Namely, change theme to edgecase.css, remove links to slid.es, remove
zoom plugin (and example slide), remove slide for viewing other themes,
change data-background examples to Edgecase colors, remove example PDF
output, and change default transition to “linear”.