Improving the Grid system

Improving the Grid system
* Desktop-first system, to match our workflow
* All views (Desktop, Tablet, Mobile) are on a 12 columns grid
* We can now use fractions (col_1_4) or numerics (col_3) to specify columns width
* We can now use fractions (col_push_1_4) or numerics (col_push_3) to specify columns push
* Rows now auto clear, so no need of clearfix everywhere and that's awesome
Adding Grid documentation

We will need to test this new grid while doing the new Arcbees website, and fix it if there's anything

@LarryMatte Let me know if you see anything that we could improve
@Pacane The Grid doc is there, if you need it