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. 
I can see why you put it here, but I actually just meant `protocol` to be for generated files. I'd rather it was `lib/impala/sasl_client.rb` or something.