Stop header from moving on mouseover in IE7

Due to a bug in the way IE applies margins on hover the header grows and
shrinks a pixel on hover. This swaps out the margin for a padding which
fixes the moving. It also adds a negative margin to the bottom of the
logo to bring it back to the same height as the search box. Otherwise
the spacing looks strange.
Merge pull request #43 from alphagov/header-wiggle

Stop header from moving on mouseover in IE7