Take advantage of the Model inheritance provided by polymodel.PolyModel to
have create a class hierarchy rooted at Person.
Also, add some missing Role classes, such as Role, (Organization) Mentor,
(Club) Member, (School) Student. Make (Organization) Administrator be a
Reviewer like Mentor is.
Patch by: Todd Larsen
Review by: Pawel Solyga, Sverre Rabbelier, Augie Fackler
Review URL: http://codereviews.googleopensourceprograms.com/607
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