Search API Location: Distance sort

Here's a patch providing Search API Location sort support.

Thanks for committing it so fast, @skek! I have found a minor issue with the previous patch: it causes a warning when $query->getOption('search_api_location') is empty. Here's one more patch against the current version.

I have found an issue with distance sort: if the search point is not set, then attempt to execute distance sorting makes Elasticsearch throw an exception:

ElasticsearchIllegalArgumentException[can't sort on geo_point field without using specific sorting feature, like geo_distance