Add a new recipe for helm-ad

I created [helm-ad](helm-ad), [helm](helm) source for Active Directory. I would
like you to add its recipe to MELPA. The recipe has passed my tests
by running [`make recipes/helm-ad`](make) and invoking `M-x package-install-file`.


Thanks,


[helm-ad]: https://github.com/tnoda/helm-ad
[helm]: https://github.com/emacs-helm/helm
[make]: https://gist.github.com/tnoda/b70a4c5304194e090479