Code for https://dtestbadge.appspot.com

Outputs an image with the current test results for a pull request using HTTP referrer. Just paste the Markdown ![Test Results](https://dtestbadge.appspot.com) anywhere on the pull request's page (in the description or a comment, for instance) and this will return an image with the pull request's current test results. It uses HTTP referrer to figure out which pull request's test results to look up.

Ideally this could be moved onto the actual autotester but using App Engine means we get SSL for free (which is necessary for HTTP referrer based redirection because GitHub is SSL only and the browser won't set the referrer when transitioning from SSL to non-SSL).