Fixed SASL QOP negotiation.
Fixed issue after Sasl negotiation for auth-conf.

Calling authenticateGSSClientWrap with a user name is only for the very
specific instance of wanting to create the final message in the Sasl
negotiation.  For all other cases the user should be left out or set to None.
Fixed wrap so that the conf_flag is passed to PyKerberos.

The conf_flag controls whether gss_wrap applies encryption to the passed in
buffer.  If the conf_flag is set to 1, then encryption will be applied to the
buffer, if it is 0, then the buffer will only be signed.  The conf_flag is the
third variable to the gss_wrap call in the MIT Kerberos library.

Also modified unwrap so that the returned conf flag is checked to see if the
server is honoring the negotiated QOP value.

Note: in order for these changes to work, changes to PyKerberos also had to be
made.
Raise an exception instead of printing a warning.
Added comment.
Merge pull request #1 from katriana/QOP-selection

Qop selection