added no_proxy handler - fixes #1318

This patch adds support for the `no_proxy` environment variable. If one is set, requested URLs will be tested against it and on a positive match the `http.proxy` option will be set to `''` (an empty string).

The CIDR matching code is borrowed from the [Zend extras library](http://framework.zend.com/svn/framework/extras/incubator/library/ZendX/Whois/Adapter/Cidr.php) and parts of the pattern class is courtesy of @hoffmann.