Added hooks for find and findOne, as well as ability to modify empty selectors

Per our discussion in the e-mail.

I suggest the following to continue working on this to make `this.userId` available to all the hooks.

1. Merge this pull request.
2. Add me as a contributor to the main repo. We'll create a separate branch for the highly experimental stuff and just coordinate more directly there. Later, I can help with the client-server tests as well.

Andrew