Small improved in performance - return keys instead return full entities.
Merge pull request #204 from gmist/return_keys_only

Small improved in performance - return keys instead return full entities
Used Flask-RESTful for API.
Fix fields type.
Improved API errors handler.
Added request.query_string value to JSON link.
Added 'check_unique_email' field to config API.
Sorted properties in CONFIG_FIELDS.