Add support for geo distance facets

This pull request adds implementation and tests for geo distance facets to tire, see http://www.elasticsearch.org/guide/reference/api/search/facets/geo-distance-facet for more details.

Thanks for your work on tire. We use it at Stuffle (http://stuffle.it) and it's great how little effort was needed to integrate ElasticSearch.

Cheers,
Robin