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
Change search_raw's return value

This makes the output consistent with that of a normal search. search_raw is now only raw in the sense that the query is raw, not the result.
Add raw_result option, revert change to REST API

Raw results may be useful sometimes, why not just give it as an option.
Update docs, fix typo
Merge pull request #87 from Treora/revise_search_raw

Revise search_raw