def grouped_markers
      markers.inject(Hash.new {|hash, key| hash[key] = []}) do |groups, marker|
        groups[marker.options_to_url_params] << marker
        groups
      end
    end
def grouped_markers
      markers.inject(Hash.new {|hash, key| hash[key] = []}) do |groups, marker|
        groups[marker.options_to_url_params] << marker
        groups
      end
    end
Returns the markers grouped by it's label, color and size.

 This is handy when building the URL because the API wants us to
 group together equal markers and just list the position of the markers thereafter in the URL.