Get markerIcon from webroot img dir

When a non-HTTP(S) string is given to the markerIcon option,
it will get the image from it's relative app/webroot/img/ path.

For example:

`'markerIcon' => 'office-marker.png'`

Would use `app/webroot/img/office-marker.png` as markerIcon. If a http/https URL is passed as argument, it will just include that like it does now.