Adding in offsets and a full width container option.

Hi Jalxob,

Thank you very much for creating this framework.  I've been using it in a project for a client, but just wanted to add in the following thing that I see is requested a lot in the forum that I use myself: offsets!

See the changes below to grid.css, I use your style and calculations to create offsets in an easy to use manner.

Please feel free to pull them into your master branch, if you find it useful. Thanks!