Feature/krb5kdc

This is a pretty big change to the current cookbook. However, I have maintained backwards compatibility with all of the previous attributes. This changes the krb5.conf template to be fully attribute-driven, which adds flexibility. I have, also, added `kadmin` and `kdc` recipes, to configure the Kerberos Admin Server and KDC, respectively.

I have included the changes from @dereklwood (a co-worker) in atomic-penguin/cookbook-krb5#3 into this pull request. This has been tested, using `bundle exec rake` on my Mac, with no issues. The plan is to pull this into our http://github.com/continuuity/loom project, to allow provisioning of secure Hadoop clusters.

Thanks