Update Sass mq() mixin to v1.1.1

- [New] Prefix `px2em` and `retrieve-breakpoint-width` functions with `mq-` to avoid potential conflicts with external modules
- [Fixed] Indentation in `mq-add-breakpoint` function

![cartman](http://24.media.tumblr.com/edf957248e009ac2bd2e5318b3694a4a/tumblr_mp2tqd0l9a1rqrt0mo1_500.gif)