LWRP for principals and keytabs

This allows for the creation/deletion of principals (in the default realm, only) via a Kerberos Admin server using the `rkerberos` Ruby gem. It also supports the creation of keytab files, using Kerberos administrator credentials. Currently, there is no support for adding/removing entries from the keytab file, which is a limitation of `rkerberos` gem. The LWRP will not stomp on an existing keytab file. This will likely require a couple iterations before it's perfect, but it seems to work for me for my use case (setting up Kerberos for Hadoop clusters).
This includes these methods into every single chef recipe, resource, and provider, which is probably a bit excessive.  Instead, they should be included by the providers.