Add minimum width for IE 8

- Set an absolute widths for header and footer on IE6/7/8
- Increase the set width for IE to 960px so it is the same width as the
  heading. Otherwise there is a white block next to the footer when the
  browser is narrow
- Add padding to the content in the footer which makes up for the width
  of the footer element being wider. This matches the approach in the
  header
- Set the background of the inner in the heading and footer to be the
  colour of their parent elements. When IE window is narrow the parent
  elements take the window width. This was causing the content to sit on
  a blank background.
Remove incorrect comment
Make footer the same width as the header

In modern browsers the header and the footer were no longer the same
size. This makes them the same. Also swapped a `media-down` usage to
`media` for future proofing.
Merge pull request #59 from alphagov/ie-min-width

Add minimum width for IE 8