Add support for primary auth plugins that implement external authentication

This change moves the code that ensures that the user is known to the
secondary auth plugin as well from checkPass() into a new method
_checkUserOnSecondaryAuthPlugin() that gets called by checkPass() and the
new trustExternal() method. The latter and the new logOff() method add the
support for primary auth plugins that implement external authentication
(eg. via cookies).
Improve method call delegation summary in README
Fix erroneous reference to authhttp in README