Reject images that are not on the assets domain or a relative path
Wrap specs relating to images in a context block
Document allowed image locations in the README
Add www.gov.uk as an allowed domain for hosting images

- There are only two, so specify them in the error message text with
  .first and .last.
Merge pull request #54 from alphagov/validate_images

Reject images that are not on the assets domain or a relative path