Define and use default http proxy

Please consider this patch.

It defines *default-http-proxy* variable to be used as default value for
proxy parameter of the http-request call, and exports it.

This is useful in particular when a library (such as, e.g., zs3) is wrapping the http-request calls without passing the proxy parameter along (which would be awkward anyway). Also, the proxy tends to be same for many requests.

I did not change index.xml/html, as I am not sure whether it is generated manually or automatically.