Breakpoint implementation, closes #160

Only off-canvas.scss has been converted so far, as a proof of concept.

Requires the Breakpoint and Sass::MediaQueryCombiner extensions — I've updated the wiki to reflect this:

- https://github.com/nys-its/excelsior/wiki/Using-Sass
- https://github.com/nys-its/excelsior/wiki/Advanced-Developer#install-ruby-based-tools
- https://github.com/nys-its/excelsior/wiki/Inner-Workings

/cc @JeffHerb @esteinborn