Fix the zoom behaviour with mouse wheel


#44

![screen shot 2013-12-03 at 4 54 13 pm](https://f.cloud.github.com/assets/2622152/1664711/367e97ce-5c2b-11e3-9e68-77f590ae6645.png)
![screen shot 2013-12-03 at 4 54 25 pm](https://f.cloud.github.com/assets/2622152/1664712/367f2ee6-5c2b-11e3-9d16-658ed0ab1053.png)
![screen shot 2013-12-03 at 4 55 00 pm](https://f.cloud.github.com/assets/2622152/1664713/3698379c-5c2b-11e3-90b4-95eeb327c397.png)