don't use the kerberos.GSSError.message attribute

Replaced the reference to kerberos.GSSError.message in any raised exception, because:

 DeprecationWarning: BaseException.message has been deprecated as of Python 2.6

and its natural repr is probably the most desirable output.
support 'success' phase correctly

When the GSSAPI mechanism's process() function is invoked for the third time (on success) it must not attempt further processing.  Instead it should clean the context and return an empty response.
Support jabberd2 SASL with really empty response

Despite http://xmpp.org/rfcs/rfc3920.html#rfc.section.6.2, jabberd version 2.2.14 cannot accept the typical "<response xmlns="urn:ietf:params:xml:ns:xmpp-sasl">=</response>".  Instead it must be truly empty, so we force an empty response for this stanza only.
Merge pull request #292 from 4gra/develop

Fix support for jabberd2 with GSSAPI