Proxy for root-relative urls

[mdanetzky]

Add a facility to proxy root-relative urls in config.
Link example:

<script src="/root-relative-script.js"></script>

Proposition:

• Determine source of request using cookies.
• Proxy such requests using a config setting like:

config["My tests"] = {
     // ...
     rootUrlBackend: "http://mydomain.com"
}