Make it work with Redmine 2.x, improve UX (see screenshot)

User should think that OpenID is the preferred way to register, which it indeed is.

English messages were taken from Stackoverflow analogous messages. (Translated them to Polish, too.)

People might need this extension while outdated http://www.redmine.org/plugins/openid-selector ranks 1st in Google for "redmine openid". Not everyone will come up with an idea to search for some unofficial updates here!