Add (opt-out) PubSub bridge from Lenny instances <=> OpenLibrary.org

[mekarpeles]

Make the default behavior (allow easy opt-out) be that each Lenny instance is able to publish an alert to OpenLibrary.org any time a book is added, modified, or removed in a Lenny instance.

This gives Open Library the ability to serve as a search engine for any Lenny instance and also lets Lenny add/create missing books in Open Library.