Changes to when buildCSS is called and minified

This takes the changes from https://github.com/chuckcarpenter/REM-unit-polyfill/pull/38 and adds the minified file. Also, the test file has been expanded to include a Foundation 5 build, so that we can test w/ larger CSS files included. Finally, I added a Gruntfile to add a few tasks, such as magnification. 

*note: jshint isn't happy at this time, that's a todo along with some unit tests.