Move OpenID box to the top, improve its messages (and move them to locale files)

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.)

![Screenshot](http://i.imgur.com/QXn08Ye.png)

I really think you should make a pull request with your work and my minor fix and send it to @jorgebg to make this official. People might need this extension while his outdated http://www.redmine.org/plugins/openid-selector ranks 1st in Google for "redmine openid".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!)