Enable the override of the default attributes

From what I understood the DAO factory adds some default attributes. In that process it adds the _id_ column with options suited to a traditional PK along with the Integer data type.

I wanted my id to have a different datatype but when I do the following:

```js
sequelize.define('picture', {
      id : Sequelize.BIGINT
})
```

it does nothing, due to the way the addition of the default attributes. So I made some changes to support that.

Let me know what you think.
Cheers