Check for WP_PROXY constants

Hi there,

While working with your (great!) plugin, I needed to override it a bit in order to make it work behind my proxy.

It’s pretty easy to do by adding proxy conf to curl_setup(), like this:

if(defined('WP_PROXY_HOST')){
    curl_setopt($ch, CURLOPT_PROXY, WP_PROXY_HOST);
    curl_setopt($ch, CURLOPT_PROXYPORT, WP_PROXY_PORT);
    curl_setopt($ch, CURLOPT_PROXYUSERPWD, WP_PROXY_USERNAME.':'.WP_PROXY_PASSWORD);
}

You might want to test and check this, but it works fine in my case.

Could be great if you add it to your plugin ??

Thanks for your work!