Option to have at most one infoWindow open at any time

I needed a way to have at most one infoWindow open at a time and figured other people might too.
If the attribute 'singleInfoWindow' on the 'google-map' element is set to true, it will inform all its children 'google-map-marker' elements to close the other info windows when being clicked.
You should define `openInfoWindowChanged` and call `this.toggleInfoWindow();` in it instead of adding it here. This is just a hack to observe the native `title` property (you can't define changed handlers for native props).