allow disabling of cache to optionally query for latest results

Frequently, you want to be able to force a full update on a normally-cached query. This PR allows you to select to cache or not with a boolean argument like:

```js
function query(fresh, done) {
  this.find()
    .sort('-createdAt')
    .limit(n || 50)
    .cache(!fresh)
    .exec(done);
}
```