clean up authentication
add new exception "BadAuthenticationType", which is raised when auth fails
because your auth type (password or public-key) isn't valid on the server.
used this as an excuse to clean up auth_password and auth_publickey so their
'event' arg is optional, and if missing, they block until auth is finished,
raising an exception on error.
also, don't close the session on failed auth -- the server may let you try
again.
added some test cases for failed auth.
Robey Pointer