Abstract layout grid CSS

[sophiedennis]

It would be good to finish off abstracting the layout CSS, setting up a set of re-usable layout classes for the grid layout. This will help a lot when we come to making the design responsive (see separate issue #4).

I have made a rather hacky start on this for the digpen IV pages (see /IV/index.php), but the homepage layout is entirely tied to IDs for layout. We need to properly set up and test at least a basic grid CSS, and apply it to the homepage in place of the current ID rules.

It may be preferable to adopt a standard responsive grid system such as 960gs or Skeleton to make it easier for others to work with the layout CSS in future.