Let the user stop and hold the page during a bounce

Fix a bug where the page abruptly jumps to the top or bottom if a user
try to stop the scrolling animation and hold it at his current 
position during a bounce.

Before : 
![bad-scroll](https://cloud.githubusercontent.com/assets/1268691/3085915/5215519a-e51e-11e3-9827-5dbdca20800a.gif)



After : 
![good-scroll](https://cloud.githubusercontent.com/assets/1268691/3085916/5ad98346-e51e-11e3-95fb-2399fa541994.gif)