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. 
might something like https://github.com/pyu10055/ruby-sasl let us avoid the nitty gritty here? It looks kinda old, so maybe not. If not, please add some comments to make this a bit easier to read.