Support retrieving suggestions from results

[Radiergummi]

Currently, there is no way to access suggestions returned from the server. This is a huge feature of Elasticsearch and shouldn't be ignored.

There are multiple approaches possible:

1. Implement a separate method for suggestions, something like $query->suggestions($conditions) which returns a special suggestion collection.
2. Overload the Collection class to use dynamic response properties.
3. Adding suggestion accessors to Collections.

For now, I'll implement variant 3 as it's the easiest. With the next major release, we'll probably adopt a more sophisticated approach.