Implement Fields parameter of search query. Closes #19.

### Summary
Implement Fields parameter of search query.

### Test / Example

```
$sherlock = new Sherlock\Sherlock;
$request = $sherlock->search();

$query = Sherlock\Sherlock::queryBuilder()->matchAll();

$request->index('foo')
              ->type('bar')
              ->fields(array('id', 'name', 'created_on'))
              ->query($query);
```