allow kerberos

[danielhstahl]

Fixes #106

Allows users to specify auth and kerberos_service_name. This won't impact existing users who upgrade.

[beckjake]

Thanks for your contribution @phillyfan1138! Can you add a unit test for this to test_adapter, similar to test_http_connection and test_thrift_connection?

I don't see how we can easily integration test this, but it would be nice to make sure we keep passing these credentials along in the future, at least.

[danielhstahl]

Sure thing! Added.

[jtcohen6]

Thanks for this @phillyfan1138!