Center ScrollLists Vertically

When a scrollList is initially created, if its vertical height is less than
the height of the viewport make sure it is centered vertically in the viewport.

How To +10/QA:
---

- Travis CI Build Passes
- Run `./init.sh && grunt qa`
- Open the scrollList demo
- Resize the window so that the first item fits fully within the viewport
- Set the number of pages to 1
- Should see that the first page is centered vertically in the viewport
- Checkout master to compare behavior

@timmccall-wf @lancefisher-wf @robbecker-wf @patkujawa-wf 
I agree that the 75 and 400 seem magical.