Add ability to connect with SASL/GSSAPI (kerberos).

This patch provides ability to connect to a kerberos-enabled Impala. Tested against Impala 1.3.1 (CDH 5.0.3).

It does add the gssapi gem as a dependency, which might be a nuisance in a non-kerberos environment, not sure what to do about that. 
this should be a separate method called `handle_challenge` or something.