Improve x-browser support for grid mixin
Update guidance
Adjust hero styles to accomodate adjusted grid mixin
Remove guidance page for grid pattern

It's got too much prominence at the moment and it's not clear what need it's serving.
Once we have a collection of patterns we can bring it back.
Merge branch 'master' into grid
Merge pull request #52 from alphagov/grid

Improve x-browser support for grid mixin