Make _build_query() accept dicts as sort values

Hi,

This pull request allows developers to use dicts as sort values in `order_by()`. This is useful to do [nested object filtering](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-request-sort.html#_sorting_within_nested_objects), which I'd like to use that in Firefox Marketplace to fix https://bugzilla.mozilla.org/show_bug.cgi?id=951626