Add student role

Student model added. It has a polymorphic association with User thus can sign on, have sessions, etc. It has some extended attributes as can be seen in the schema and specs.
Yes, well technically it is just an association, but this association can happen exist between multiple models.

For example, if I create a new student `bob`, and sign up with user params, any time I execute `bob.meta` it will return his user info (email and password).