First pass at shared_folders endpoint.

Initial implementation of the new shared_folders API endpoint presently in beta. It works, but may need tweaks prior to landing. Opening pull request to start a discussion/critique.

- [ ] Style review. No method overloading makes this ugly. Also, my first time "writing" CoffeeScript.
- [ ] Documentation. I tried to match the existing style.
- [ ] Code order. Where in the library would you like this?
- [ ] If the first three check out, tests will be added in a subsequent commit on this PR.

Other things I missed? (No feelings will be hurt.)