Move onboarding app to Tornado

Two reasons for this. First, this removes flask and its dependencies, which are
quite sizeable. Second, pyinstaller now barfs on simplejson, which is a Flask
dependency. I just don't have time to fix this upstream, so doing what we
should be doing anyway is a no-brainer.
Merge branch 'master' of ssh.github.com:mitmproxy/mitmproxy