Add framework for testing stylus mixins

I've added in https://github.com/jasonkuhrt/stylus-test-runner to start testing our stylus. To make it work, I've started testing the em mixin in the unit mixins file. I'd like us to be confident in the mixins we are building, and I believe tests are the way to go with this. 

Test runner is using stylus 0.32.X. We can fork the repo and update considering the runner hasn't been updated in a year.