add zoomToMarkers functionality

if there are more than one marker in a group, zoom to them, by calling $('.selector').jHERE('zoomToMarkers'); which will change the viewport to cover all markers on the map. This functionality is also shown in the markers example.

Additionally there is a function $('.selector').jHERE('zoomToMarkers', true);  which keeps the current center but zooms the map out, to also show all markers on the map.

Also fixed a hint issue.