Added field to custom validator functions

When defining custom validators, the field is never added. This makes it difficult to return proper error messages, especially since [in the DAO](https://github.com/sequelize/sequelize/blob/master/lib/dao.js#L234-L235), it doesn't return the field if its a custom function:
```javascript
if (!fn_msg && !is_custom_fn)
  err += ": " + field
```