Fix nested scrolling issue in Firefox

This PR fixes #16 which reported an issue with **tinyscrollbar** in Firefox 28 on OSX (possibly elsewhere) where the scrolling area would scroll at the same time as the window would scroll, creating a strange, undesirable parallax effect.

#### To replicate the issue
On FF 28 on OSX, create page which scrolls normally, i.e. the body is taller than the window. Then create a tinyscrollbar div any size. Scrolling the tinyscrollbar div should scroll while the page also scrolls.

#### The fix
I used the newer `wheel` event over `mousewheel` or `DOMMouseScroll` where available and normalized the `event.deltaY` value for consistent cross browser scrolling. MDN has some great information for reference [[source](https://developer.mozilla.org/en-US/docs/Web/Reference/Events/wheel)]. We normalize the `event.deltaY` value with a fallback to `event.wheelDeltaY` and `event.detail` when necessary.

#### Thanks
Thanks for creating this scrollbar plugin, it's pretty awesome! After looking into and testing others, this was a great choice with the small size and browser support that we were looking for. Hoping to contribute any future fixes we come across!