Add a Minimum scroller height/width

See this [screenshot](http://i.imgur.com/bbhJZ.png). That tiny little black thing at the right side is antiscroll.

A simple fix is to add a minimal height, and adjust the top position based on it:

``` javascript
// Scrollbar.Vertical.prototype.update

height = trackHeight * paneHeight / innerEl.scrollHeight
top = trackHeight * innerEl.scrollTop / innerEl.scrollHeight

if height < 30
top = top - (30 - height)
height = 30

this.el
.css('height', height)
.css('top', top)
```

Minimum height should probably be an option, as well.

Start at Scrollbar.Vertical.prototype.update, which the issue identifies, and locate the corresponding horizontal scrollbar update for the width behavior. Confirm how scrollbar dimensions are calculated, then verify that minimum height and width are configurable options and that both scrollbars keep the intended positioning.

Written by the indexing model from the issue text.