add proxy support to native fetcher

Example:
    pagespeed FetchProxy        127.0.0.1:8081;
    pagespeed FetchProxy        www.proxy.com:8081:

By replace the ip address which nginx connect to with the ip address of the proxy server and replace the dns name nginx take to resolve with the dns name of the proxy server, we manage to pass all the fetch traffic to the proxy server. BTW, i am not very familiar with http proxy and github, so careful review is needed ^_^