Revise search_raw

As described in #86, all query transformation stuff is moved from `_Model` to `store.py`, making `_Model.search_raw` a thin wrapper around doing a Elasticsearch search; the only help being that the index name and document type are filled in for you.

Annotation.search_raw like before adds a filter to the query, to comply with annotation permissions. I added the option to disable this authorization filter.

> It looks like the error case behaviour here was changed. Do we want to return `e.result` like in the old code or allow exceptions to bubble?

The funny thing is that ElasticsearchException does not even have a `result` property, so I'm not sure what the old behaviour was supposed to be. :)

I opted for just returning Elasticsearch exceptions. Abstracting them seems like unnecessary complication, as raw searches are very ES-specific in any case.