Adding vehicle-tax.html.erb file and assets

This is a copy of the existing tax-disc file and assets with a couple of modifications - replacing 'tax-disc' with 'vehicle-tax' in the CSS.
updating root_controller and application.css to include vehicle-tax
add some tests
update the transaction_rendering test not overwrite it
Merge pull request #699 from alphagov/adding_vehicle_tax_page

Adding vehicle-tax 'exception' page