Move query transformations to store.py

The _Model.search_raw method now becomes just a simple interface to
elasticsearch's search.
Pull error handling out of ES.search_raw

Errors are to be handled by the caller. In principle, search_raw is just
a thin layer covering Elasticsearch.search to insert the index name and
document type.
Add a test for Annotation.search_raw
Allow overriding es.authorization_enabled
Add a docstring