Change mousewheel delta calculation logic.

Hello,

Recently, we moved to jquery-mousewheel 3.1.9, and it changes the way calculating the deltas. So I changed the logic a little before, but it made the scrolling extremely slow in some environments. So the change was reverted.

I just found that there's a value called `event.deltaFactor` in jquery-mousewheel, and I re-change the logic using `event.deltaFactor`. To prevent the problem like before, I hope to test it in many environments before applying it into the master branch. The change is tested with my Macbook Trackpad and Logitech Mouse in the latest Safari and Chrome. If there's no problem in other environments and seems good, I'll merge the patch.

You can test it with the example, `examples/options-wheelSpeed.html`. The terminal commands will be like this:

```
git clone https://github.com/noraesae/perfect-scrollbar.git -b mousewheel-speed
cd perfect-scrollbar
open examples/options-wheelSpeed.html
```

If there's any problem, please report it with comments. Any further contribution also will be welcomed.

Thanks in advance.