Acceptance tests

Fixes #315.
This is really annoying. There are surprisingly few people complaining about this. Here are some tickets:
 * https://bitbucket.org/birkenfeld/sphinx/issue/823/i-wish-download-would-keep-the-paths-not
 * https://bitbucket.org/birkenfeld/sphinx/issue/459/download-files-included-multiple-times-in