Fix List scrolling issue

A user had to target the Scroller and not the List if it wanted to define the size of the scrollable viewport.
With this change the user can directly target the List.
Fix the size of the knob in Firefox.

Fixes #38.
Merge pull request #38 from NV/slider-firefox-bug

Slider: the knob gets cut off in Firefox
Merge pull request #36 from aadsm/list-scrolling

Fix List scrolling issue
Add vertical slider
Manually land #39 "Implement vertical axis slider".
Update Slider to use only 2 elements
Update Montage dependency in package.json
Merge pull request #45 from romancortes/2-elements-slider

2 elements slider
Updates Slider README

to reflect "vertical" and "2 element" changes
Merge pull request #46 from simurai/slider-readme

Slider README
Move default value code to AbstractSelect
Fix undefined item in content breaking drawing
Merge pull request #54 from aadsm/select-undefined-item

Fix undefined item in content breaking drawing
Merge pull request #53 from aadsm/select-default-value

Move default value code to AbstractSelect