Fix #211, #217: Remove support for auto-login/logout.

Have onlogin callback store the assertion and call navigator.id.logout
immediately. Then, have the onlogout callback check for the stored 
assertion and resolve any post-login deferred we have set. What this
does is it immediately logs out users from Persona once they log in,
which removes the possibility of automatic login being triggered on
another page.

Automatic logout, in turn, only does anything if an assertion has been
stored, which means automatic logout requests do nothing as well.