support SUM(col1*col2) AS col1

With this patch, user can write sum clause like below:
```javascript
Model.find()
  .where(...)
  .sum({sum: 'col1*col2', as: 'col1'})
  // OR
  .sum([{sum: 'col1*col2', as: 'col1'}, {sum: 'col3/col4', as: 'col3'}])
  .exec(function (err, models) {
    ...
  });
```