Added User, Ad, Comment models and configuration

to run initial migration do:
   $ python manage.py db init
   $ python manage.py db migrate

to run interactive session:
   >> from adengine import create_app, db
   >> app = create_app('default')
   >> app_context = app.app_context()
   >> app_context.push()
   >> db.create_all()
   >> from adengine.models.base import User
   >> user = User(email='john@example.com',
   ..             username='john', password_hash='cat',
   ..             confirmed=True)
   >> db.session.add(user)
   >> db.session.commit()