text: Title: ManyToMany for SQLAlchemy
  Body: I've trying to get M2M field working with SQLAlchemy. Following the documentation
  
  ```
  class LandingPageFactory(alchemy.SQLAlchemyModelFactory):
      FACTORY_FOR = LandingPage
      FACTORY_SESSION = db.session
      name = Sequence(lambda n: u'Landing Page %d' % n)
  
  
  class LPRotatorFactory(alchemy.SQLAlchemyModelFactory):
      FACTORY_FOR = LPRotator
      FACTORY_SESSION = db.session
      name = Sequence(lambda n: u'Landing Page %d' % n)
  
      @post_generation
      def landing_pages(self, create, extracted, **kwargs):
          if not create:
              return
  
          if extracted:
              for landing_page in extracted:
                  self.landing_pages.add(landing_page)
  ```
  
  Then if I try to set it up like so 
  
  ```
  lp1 = LandingPageFactory()
  lp2 = LandingPageFactory()
  db.session.commit() # All good here
  
  lpr = LPRotatorFactory(landing_pages=(lp1, lp2))
  db.session.commit() # This throws the error
  ```
  
  This throws an Attribute error.
  
  ```
  self.landing_pages.add(landing_page)
  AttributeError: 'InstrumentedList' object has no attribute 'add'
  ```
  
  I noticed all the docs and examples use Django, but didn't see anything too specific. Am I doing something wrong?
  
  Thanks
  