add support for HTTP Proxy Auto Configuration PAC files

We are behind a company proxy, and hence we need to maintain HTTP proxy settings (under Administration > Server). Naturally, there is a lot of internal communication going on (more than external) so we also need to maintain a list of Non Proxy Hosts. However, this is tedious and error-prone. The list is unstable and necessarily incomplete.

It would be much better if we could make use of the company PAC file, which can decide, dynamically, whether routing to a particular host requires going to the proxy or not.

All about PAC files: http://findproxyforurl.com/