Added a "credsfile" option to shiva.conf.

This option allows the user to provide several credentials which
will be the ones allowed when "authenabled" is True. The option's
value should be a path to a file which uses the following format:

user1:password1
user2:password2
...

SMTPReceiver has been changed to handle this option, and
CredentialValidator now uses a set of username and password tuples
to store the credentials.