Adding dropping animation loop

I can't seem to find how to make looping bounce animation like the one in Google Map Marker.
https://google-developers.appspot.com/maps/documentation/javascript/examples/full/marker-animations

So I created my own for my personal use. I just want to share this patch, just in case someone else also needs it.

To start the loop, all I need is to add a new field `loop` to the object.

```javascript
marker.bounce({duration: 500, height: 100, loop: true});
```

and to end the loop, I execute this.

```javascript
marker.bounce(null);
```

My implementation doesn't execute the callback function when the bouncing ends. Well, since the stopping is executed manually through `marker.bounce(null)`, I don't think putting callback will be that useful because we can call any function after we stop the bouncing.