Use sass for development and add build system

I've added a build system for auto transpilation / minification of the library. I decided to port the library to sass because it should help to ease future development, allow a cleaner code base and also allow additional functionality in the future ie. implementing mixins for the parts of the library that aren't semantic (grids / nav) for users who care strongly about eliminating non-semantic dom elements. It can also make it more easily customizable by simply tweaking variables (much as bootstrap does).

I've done very little work on the actual scss source itself other than chopping it up, but as a start I've added variables to allow adjusting the responsive breakpoints