Automatically append slash to unmatched requests

Right now, if you call http://localhost:6543/fouille/ you will get a 404. But http://localhost:6543/fouille (without a slash at the end) will work.

This PR changes the mapping, so that by default every route should be called with a slash at the end. But if the slash is not given (and no route can be matched), a slash will be appended automatically to the url.

This PR is based on #18, so #18 should be merged first.