Create new grid helpers for creating grid layouts

Adds a standard mixin and extendable selectors which let you create a
fully responsive and cross browser grid system.

The grids use fixed pixel widths for older versions of IE and use
border-box for new browsers. This lets us remove the need for a padding
div and just have the column divs and a div to contain columns.

It also reduces the width of the standard site wrapper so that as a
default you can just put elements on a page and not need to indent them
with padding blocks. This means for a minimum viable page you would need
less markup.
Minor quibble: It's likely to be applied to our `#wrapper` element, so makes sense, but otherwise container feels like a more widely used term (you did ask for naming pedantry)

- ref: http://www.stuffandnonsense.co.uk/archives/whats_in_a_name.html