Move over to sass-mq and update to v3.x

- [deprecated] function `mq-add-breakpoint`. Becomes a mixin (see below)
- [new] mixin [mq-add-breakpoint](https://github.com/kaelig/sass-mq#adding-custom-breakpoints): `@include mq-add-breakpoint`
- [new] Test compilation with Ruby Sass and Libsass in Travis-CI
- [new] [Sass documentation](http://kaelig.github.io/sass-mq/) written with [SassDoc](http://sassdoc.com/)
- [new] MIT License, more permissive
- [fixed] Move all the references to older repos to the new sass-mq/sass-mq one