Version 2

Complete refactor of the grid system.

Created the _grid mixin that generates all the grid classes including push and pulls. Added helper mixins for widths, pulls and pushes. Added modifier mixin.

Divided into 3 files
* grid-base.styl (Mixins)
* grid-config.styl (Configuration)
* grid.styl (Implementation)

Let's keep this pull request opened to discuss the new changes and make improvements to code / documentation before merging.