Update the generated migration skeleton for async

The auto-generated migrations don't mention done which is confusing if you're new to sequelize and you've got a background in Rails.

This branch changes the migration skeletons generated by the sequelize executable. Fixes issue #566.

Before it was:

```
module.exports = {
  up: function(migration, DataTypes) {
    // add altering commands here
  },
  down: function(migration) {
    // add reverting commands here
  }
}
```

and this change will make it:

```
module.exports = {
  up: function(migration, DataTypes, done) {
    // add altering commands here, calling 'done' when finished
  },
  down: function(migration, DataTypes, done) {
    // add reverting commands here, calling 'done' when finished
  }
}
```