Add AUTHZID support for PLAIN mechanism.

This change adds support for authenticating for a different authorization identity (AUTHZID) using the normal username (AUTHCID) and password attributes, for the PLAIN mechanism. It adds a single attribute called `identity` to the constructor, which is empty by default (corresponding to 'same as AUTHCID' in SASL PLAIN).

See also: http://www.ietf.org/rfc/rfc4616.txt, section 2.