remove percentage sign when value is 0

I added a jeet-get-percentage function which checks if the value is 0, when it is 0 it just returns 0, otherwise it returns the value as a percentage value. this function is now used on all relevant values in the grid.

all changes happend in sass and stylus

Codepens:
 * sass: http://codepen.io/di5abled/pen/ZYpqbP 
 * stylus: http://codepen.io/di5abled/pen/MYjPaL