Update OpenIdAuthentication to use Ruby OpenID 2.x.x

The new Ruby OpenID library has been released and it has a bunch of
major changes from 1.x.x. This patch updates the OpenIdAuthentication
plugin to use the 2.x.x version.

I wouldn't usually feel much pressure to upgrade, however the ruby-
openid 1.x.x gem is broken after Ruby Gems 1.0 was released.

I think we should also consider freezing ruby-openid in this plugin so
this type of thing doesn't happen again.

http://dev.rubyonrails.org/ticket/10604