Check for prior querystring on session initiator when adding auth_method
fix Auth/CAS.php getSessionInitiator()

changed hard-coded "Shiboleth" to "CAS"
checking for prior querystring before adding auth_method
clarified number of choices ChoiceAuth can handle
changed private method (authUser) to protected
efficiencies in Auth Manager

- new object to hold currently proxied auth object so it isn't
  re-created unless it is reset.
- helper method for getAuth to be more DRY
shortened some param retrievals
Fix broken Shib logout

logout failed when requested from a page with the auth_method
querystring param