Zooming in/out the map updates zoom value. Fixes #59.

Listen for `zoom_changed` events and update the map zoom level if new value is not equal to the old value.
Not really. Google Maps ignores the request if the new value is the same as the current value. I just have a habit of checking the `new != old` before setting values, incase implementation doesn't handle it or changes.

I can remove the check if you feel it's unnecessary.