HYDRA-478: Fix ScrollList

**SUMMARY**
The ScrollBar component behaved incorrectly when the scale of the scrollList changed. See this video:  http://screencast.com/t/TKJlF0Ide

**SOLUTION**
The `this._scale` information was changed from the absolute scale of the `ScrollList` to the relative scale. The `ScrollBar#_updateScrollBar` and `ScrollBar#_placeScrollBar` methods were updated  so that they would still behave consistently when the user zoomed in or out. Additionally, in the call to `ListMap#transform`, the `scale` parameter was changed from `1` to the appropriate scale, so that the scrollList wouldn't be set back to a zoom of 1 when the ScrollBar was dragged.

**TESTING**
Pull this branch of `wf-uicomponents` and do `grunt dev && grunt qa`. Open the ScrollList demo. Try the scrollbar while at no zoom, zoomed in, and zoomed out. Behavior should be consistent.

**CR**
Please review: @lancefisher-wf @timmccall-wf 
FYI: @patkujawa-wf