FIX - Sass Cleanup/Update

This work involves some spring cleaning around this site's Sass/CSS including:

* updating Bourbon/Neat versions and moving copies of each to the local assets/sass/utilities directory
* removing references to the unused Symbol Set icon typeface
* correcting a compilation syntax around a print-based media query in _print.scss
* making all Sass extends used in the project more efficient by switching to Sass placeholders
* updating readme with Sass compilation/watch instructions.