Use cURL as alternative to allow_url_fopen in installer

The installer tests currently require allow_url_fopen. Alternately, if cURL extension is available it should use that instead via an installer_get_url($url) function or something to that effect. If neither is present, then the tests can't proceed, but at least we tried everything we could do, and you can still skip over the tests anyway. :)

Example cURL usage:

http://wiki.dreamhost.com/index....

On PEAR, there is a HTTP_Request package that does it too, and that is more compatible across systems. Maybe could it be considered.