Issue #220: Fix large scroll steps when content is long

[MihaiBalint]

If the content-to-viewport ratio is very high then the bar should be tiny and the scroll step should be normal. However the delta formula does not consider the fact that the bar height is restricted to a minimum height (for usability) and incorrectly increases the scroll step length with the content size.

With large content (in my case it was 150 000px) and default options, the large step size makes scrolling very coarse to the point of unusable.

[MihaiBalint]

I believe this also fixes #233 and possibly also #211

[kasperoo]

2 thumbs up for this, you have fixed the scrollbar updating the position when using the scrollBy method for me. (essentialy #233)

[r3verser]

@MihaiBalint nice fix bro!

[MacMaru]

I ran into large differences between browsers, has anyone looked at http://stackoverflow.com/questions/5527601/normalizing-mousewheel-speed-across-browsers