Support for messages and logout endpoint

Signo can display various notices that RP asks for. Also Signo is meant
to be logout endpoint. It does not redirect back to RP after logout
anymore. It now supports expired sessions for Foreman and redirect back
to URL from where user came from if he is already logged in.
Also fixes some test issues.

See http://projects.theforeman.org/issues/2502 for more details