Added support for `geo_distance` facets

Usage:

Tire.search 'venues' do
  query { string 'pizzeria' }

  facet 'geo' do
    geo_distance :location,
                 {:lat => 53.54507, :lon => 9.95309},
                 [{:to => 1}, {:from => 1, :to => 10}, {:from => 50}],
                 unit: 'km'
  end
end

Closes #701