Fix value of 'speed' when set default is 'auto'

I fixed problem about 'speed' when set default it is 'auto'.
+ Old logic: 
   speed very slow when move scroll has delta is small. But it very quickly when go to bottom to top because it has value is 0.
+ New logic: 
  speed calculate by delta of scroll move. So it always run normally, not quickly/slow when move scroll.
See test: http://svbuichu.com/javascript/ipad-test/

Other problem I found:
Now, when I used it on Ipad2, sometime, it do not work when click next action to scroll.
@travco, it's actually good as is. `speed` is already declared within that scope, so no need to do so again.