title: Foreign keys with Sequelize are not created
body: <p>I use Sequelize for my Server (with mysql dialect); in Sequelize's documentation is written that this:</p>
  
  <pre><code>var Task = this.sequelize.define('Task', { title: Sequelize.STRING })
  , User = this.sequelize.define('User', { username: Sequelize.STRING })
  
  User.hasMany(Task)
  Task.belongsTo(User)
  </code></pre>
  
  <p>creates automatically foreign key references with constraints;
  but for me this doesn't happen:</p>
  
  <pre><code>var Shop = sequelize.define('Shop', {
      name: Sequelize.STRING,
      address: Sequelize.STRING,
      phone: Sequelize.STRING,
      email: Sequelize.STRING,
      percentage: Sequelize.FLOAT,
      text: Sequelize.TEXT,
      categories: Sequelize.TEXT,
      start: Sequelize.DATE,
      end: Sequelize.DATE
  });
  
  var Offer = sequelize.define('Offer', {
      name: Sequelize.STRING,
      deadline: Sequelize.DATE,
      optionDuration: Sequelize.INTEGER
  });
  
  Shop.hasMany(Offer);
  Offer.belongsTo(Shop);
  </code></pre>
  
  <p>This creates the two tables shops and offers, both of them with only "id" primary key</p>
  
  <p>I also have some n:m associations like:</p>
  
  <pre><code>Group.hasMany(Accesslevel);
  Accesslevel.hasMany(Group);
  </code></pre>
  
  <p>but also in this case, in the join table that Sequelize creates, there are no foreign key;
  so if I delete for ex. an acccesslevel, than the corresponding records in the join table accesslevelsgroups are not deleted.</p>
  
  <p>Does anybody know if I'm doing something wrong or missing something?
  What I need is to create all the foreign keys for the associations and the possibility to specify the behaviour 'onDelete' and 'onUpdate' (cascade)</p>
  
  <p>-- UPDATE
  I've created a route for executing sync:</p>
  
  <pre><code>myServer.get('/sync', function (req, res) {
      sequelize.sync({force: true}).success(function() {
          console.log('sync done');
          res.send(200, 'sync done');
      }).error(function(error) {
          console.log('there was a problem');
          res.send(200, 'there was a problem');
      });
  });
  </code></pre>
  
  <p>So then in the browser I type 127.0.0.1:port/sync to create the db structure</p>