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).