Better support building passenger with RVM and chef-rvm

I don't think this pull request is in its final form, but I was hoping it might serve as a jumping off point for getting some feedback as to what you'd like to see in terms of better integrating this cookbook with chef-rvm.

Clearly not an issue with this cookbook, but better integration would make situations like https://github.com/fnichol/chef-rvm/issues/249 easier to handle. This is very RVM specific so rbenv would still suffer from the same problem, so ideas on handling both in a more generic way are welcome.

I'm fairly certain I'm up to date on my Opscode contributor paperwork.