Implemented rfe #1416 config.inc.php proxy settings for version_check.ph...

In this pull request I attempted to implement the possibility to add proxy settings for the version check feature, as requested in rfe #1416.

I did not add the proxy settings to the user preferences, but I only included them in the setup script, since these should be server-wide.

Theoretically, the implementation allows to, optionally, specify a user name and a password to be used for basic authentication with the proxy. However I was unable to test this scenario. I only managed to test with transparent proxies.

And lastly, my server (apache2/ubuntu) seems to hang when the url of the proxy server is invalid. I wonder though, if it's a setting on my server that is causing this. Can someone confirm the behaviour after applying the patch, logging in and refreshing the main page?