replace inner property scrollDuration to public and allow users to modif...

...y and control the scroll animation speed

Hi
I found there is no method to control the scroll animation speed for iCarousel.
After little research, it seems change the inner property scrollDuration to public and give some default value would be fit this need, and it works fine on my project now.
If you like it, please merge it :)

And thanks for iCarousel, it is awesome