Add image for the new (high res) GOV.UK logo

This logo has a higher resolution than the logo that's currently
available and includes the underlined hover state.
Set the new high-res GOV.UK logo for all 4XX pages

Added a slight fix for oldIE because of how it deals with margins.
Use Helvetica Neue as the default font stack

After some discussion with a few designers it was decided that we
shouldn't send the font across for the redirector error pages. It
would increase the payload for the pages by ~300kb for a single load.

Setting the weight to 400 for better legibility.
Merge pull request #161 from alphagov/use-high-res-logo

Use high-res logo and simpler font stack