allow imagePath to be a function, useful for cache busting

[jschr]

Adds support for a function to be provided as the imagePath option. The current option doesn't easily allow for cache busting assets.

Usage:

var markerCluster = new MarkerClusterer(map, markers, {
    imagePath: function (i) {
      // would return something like m1-e359da86cb.png
      return imageCache['m' + (i + 1) + '.png']
    }
});

[PeterTheOne]

Haven't tested it but it makes sense and looks good!