add kerberos authentication support

address #8
This pull request adds kerberos authentication support.

While writing this patch, I referred to the following pages.

* https://github.com/zenchild/gssapi/wiki
* http://python-notes.curiousefficiency.org/en/latest/python_kerberos.html

As mentioned in #9, in my environment, this [test](https://github.com/shoichikaji/webhdfs/blob/test/test.rb) passed.