Allow HTTP authentication without password.

This makes it the plugin useful for any form of webserver-based
authentication (CAS, SAML, etc.); of course, other plugins will
have to be installed to handle connections to the IMAP/SMTP
servers, but at least they don't need to duplicate the basic
HTTP authentication functionality.
Merge pull request #141 from rasky/http_without_pwd

Allow HTTP authentication without password.