Added the ability to login via OpenID.

* Refactored AccountController#login to use either
  password or openid based authentication
* Extracted AccountController#successful_authentication
  to setup a user's session cookies and redirect
* Implemented the start of AccountController#open_id_authentication
  which will check with the OpenID server and perform authentication.
* Added text field for the OpenID url to /login
* Added identity_url for OpenID to the user forms.
* Added option to login with OpenID to the register form.
* Added a root url route, which is used by the OpenID plugin

  #699

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@2442 e93f8b46-1217-0410-a6f0-8f06a7374b81