Change default zoom delta and animate mousewheel zoom

This PR basically suggests three changes:

- Change the default resolution constraint to snap to the OSM resolutions instead of having intermediate zoom steps between the OSM resolutions. See #219.
- Animate zooms and pans triggered by the mousewheel interaction.
- Debounce mousewheel events for better performance and preventing very fast zooms when using the mousewheel and the trackpad.

These changes can be tested on http://erilem.net/ol3/examples/full-screen.html?renderer=canvas.

Please test and review.