add http:// if no protocol is present.

If someone types in a url of the following format www.google.com. Then it's relative to the current url e.g. http://www.example.com/www.google.com would be the example. The following url is also valid /resource-to-view which would result in http://www.example.com/resource-to-view. Also, //www.google.com is also valid.