Simple grid w/push options

I've added "push-" classes in order to centralize an element without the need of creating blank elements on the page.

Like instead of using:
<div class="col-1-3"></div>
<div class="col-1-3"><!-- Centralized content here --></div>
<div class="col-1-3"></div>

We can now use:
<div class="col-1-3 push-1-3"><!-- Centralized content here --></div>
Merge pull request #19 from mvsaturno/patch-1

Simple grid w/push options