Make it compatible with node-sass 1.2.3 (libsass)

* Drop Gemfile
* Update grunt tasks to use grunt-sass
* Avoid the duplicate application.sass for Bower
* Drop sass-list-maps polyfill, use native Sass 3.3 hashes
* Remove globbing (Rails apps will change application.sass manually)
Remove Bourbon as BEMO dependency

* Remove vendor prefix mixins (Autoprefixer FTW)
* Import some useful helper functions and mixins:
  - clearfix
  - size
  - position
  - triangle
  - inline-block
Add grunt-autoprefixer task
Edit .grid__item width setter syntax

* width--3-12 becomes width-3-12
* desk--3-12 becomes desk-3-12
* desk--push-3-12 becomes desk-push-3-12
* desk--pull-3-12 becomes desk-pull-3-12

pick
Uniform grid column syntaxes
Change breakpoint names to facilitate mobile-first approach

* gte-lap becomes simply lap
* lap becomes eq-lap
Remove TODO