+ Added aurweb.models.user.User class. This is the first example
  of an sqlalchemy ORM model. We can search for users via for example:
  `session.query(User).filter(User.ID==1).first()`, where `session` is
  a configured `aurweb.db.session` object.
+ Along with the User class, defined the AccountType class.
  Each User maintains a relationship to its AccountType via User.AccountType.
+ Added AccountType.users backref.

Signed-off-by: Kevin Morris <kevr@0cost.org>