Clusterer icons are missing due to old URL

Hi,

I've noticed (yep, some time since last time I used this package!) that clusterer icons are not loading ok due to urls pointing to old repository (on Google Code I think). I was thinking about making a PR for updating the library but, for now, it can be fixed quite easy setting the `imagePath` option to `'https://github.com/googlemaps/v3-utility-library/raw/master/markerclusterer/images/m'`, like this:

```javascript
cluster = new MarkerClusterer(GoogleMaps.maps.zoneMap.instance, ms, {imagePath: 'https://github.com/googlemaps/v3-utility-library/raw/master/markerclusterer/images/m'});
```
So, this is issue is solved and can be closed right now, but just for people looking into this issue as I did at first :)

Start by locating the package's MarkerClusterer configuration and the old image URL described in the issue. Compare it with the supplied imagePath URL and verify that clusterer icons load without requiring consumers to set the override; no test file is mentioned.

Written by the indexing model from the issue text.