Support of proxy with authentication

Hello,

First of all, thank you for this module!

I experienced problems with recaptcha behind a proxy server.

I used the post from the site: http://www.greenhughes.com/content/recaptcha-drupal-and-proxy-way-make-i...

I adapted it to use the drupal_http_request method (see http://drupal.org/node/863582).

Note:
- You also need to patch your common.inc to support the proxy server with drupal_http_request
- You need to define the following variable (depending on your environments) either in the settings.php or in the database, as you want.

$conf['proxy_server']   = 'PROXY_URL';
$conf['proxy_port']     = 8080;
$conf['proxy_username'] = 'USERNAME';
$conf['proxy_password'] = 'PASSWORD';
$conf['proxy_skip_selftest'] = FALSE;
$conf['proxy_exceptions']  = 'localhost,127.0.0.1,';