order by with raw SQL

Very similar to #304 , it would be nice to be able to order by some raw (but properly escaped) SQL. My case is:

``` javascript
    cities.find().order("ST_Distance(geopoint, ?)", [point]).limit(1);
```

Two problems with this; first, it doesn't take ? and a list. Second, even a properly formed query gets quoted like it's an identifier.
