decided to ditch saving the docs in git
ignore YARD generated docs in /doc
Reworked persistent objects into the new Models module

- Users can be tagged with string specifiers called Tags
- Justbot::Helpful::ClassMethods#define_tag for plugin authors
- Twitter information resides in its own independent model
- User admin status stored as a Tag attribute on User
- Limited reorganization of method definitions, such as removing the
    utility methods in Justbot core.