add option to use PPA repository instead of official nginx repository

I had trouble getting the official repository to work. I'm not sure what the root problem was, but the PPA repository worked so I figured it would be nice to provide an easy way to use it. Also, setting the `repository` attribute to nil should skip the repository setup (might help someone).

Valid options for the `repository` attribute are:
- `official` original nginx repository
- `ppa` volunteer ppa repository