Qop selection

I needed to provide three fixes:  First, I needed to fix the negotiation of the qop between the client and the server.  I went with the option of composing the final sasl negotiation message in pure-sasl instead of letting PyKerberos do it because the code for this negotiation is a SASL thing and thus belongs in the library providing SASL, not the library providing generic Kerberos functionality.

Next I needed to fix the wrap and unwrap calls (there was a syntax error).

Finally, in order to allow the client to choose auth-conf, I needed to pass a flag into the gss_wrap call in PyKerberos to indicate that the message should be encrypted as well as integrity protected.  This final fix required changes to both pure-sasl and PyKerberos.