1.4.0 Upgrade

This is a backport of @josh's work on our camo install for GitHub. The StreamLimiter stuff that was introduced in #34 failed to run reliably in production so we've reverted it. We've also removed the host blacklisting feature since it's not a concern anymore since we now run on heroku.

The two big improvements over the old versions are:

* No longer forward client ip headers to third party systems. This avoids leaking information in things like README's to 3rd parties building advertising platforms.
* Proxy SSL images instead of returning an http 301. Hosts that fail to verify ssl will 404 which is the same behavior that chrome and firefox adopt currently.

/cc @josh am I forgetting anything?