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.
Fixes log.debug message in _connect_proxy
Merge pull request #292 from 4gra/develop

Fix support for jabberd2 with GSSAPI
Merge pull request #294 from mofrank/develop

Fixes log.debug message in _connect_proxy
Fix own_host in ping plugin
Only request auto-receipts for messages with bodies
Bring back use of dnspython for A/AAAA resolution.

This is behind a use_dnspython flag, however, so it can be disabled as
desired.
Make ssl args work in Python <=2.6.4
Bump to 1.3.0