Clusterer grid size configuration

For the Clusterer marker manager (clusterer2.js by Jeff Poskanzer), I have added the option to set the grid size within the GMap configuration interface. I realise GMap is probably not accepting new features right now, but thought this might be useful for anyone working on a legacy map.

The effect is to increase or decrease the density of cluster markers displayed on the map.

The patch just adds a configuration field alongside the others (gmap_settings_ui.inc), then sets the obj.clusterer.gridSize parameter when clusterer is initiated (js/clusterer_marker.js).