geo distance filter unit should convert to "distance_unit"

The geo_distance filter should output "distance_unit"
for mi/km instead of "distance_type". "distance_type" is for
selecting between arc and plane. Described here:
http://www.elasticsearch.org/guide/reference/query-dsl/geo-distance-filter/
Merge pull request #225 from sycora/master

Fixed bug in geo distance filter. Should use "distance_unit" not "distance_type"