Feature/resource set

@barnabyalter I need to make the rake tasks work, but figured I'd open the PR to get some feedback/start a conversation sooner than later.

@NYULibraries/hydra please have a look and provide feedback as you see fit.
@scotdalton This was the method because @chrpr had some concerns about `.destroy` doing the trick. But it definitely does. `Nyucore` is an `ActiveFedora::Base` so its destroy method is `https://github.com/projecthydra/active_fedora/blob/master/lib/active_fedora/persistence.rb#L71-L94`. I think it's safe to remove this line for good.