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'm not a huge fan of a long line with `unless` hidden at the end. Please split this out into a block, like

```ruby
unless ['PLAIN', 'GSSAPI'].include? @sasl_mechanism
  raise "Unknown SASL mechanism: #{@sasl_mechanism}"
end
```

Also, I don't think string interpolation will work with single quotes.