OpenID support

[sebnow]

Support OpenID for authentication. This is more of a convenience than required functionality, so it is low priority. In the grand scheme of things it would be nice to have implemented since Archlinux has many different authentication systems (flyspray, punbb, aur, mailing lists, etc). Supporting it in AUR won't help until it's supported in other systems as well, but it's a step in the right direction.