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
Add a homepage and return the README at /readme

As part of making the app friendlier more Hypermedia-y. It also follows the
guidance in the Service Manual:
https://www.gov.uk/service-manual/making-software/apis.html#document-by-discovery-and-example
Move json_examples to /public for linkability

We want to make these easily available over HTTP so that they can be linked to
from live documentation.
Rewrite public links in README to work over HTTP

The README is written for GitHub, where they work as-is, but we want them to
work when viewing the README in the running app.
Tweak README to better display allowed markup

This also improves it when rendered by the app (as opposed to GitHub).