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 
`400` seems like a magic number for these tests because it's far away from its declaration. Personally, I'd prefer to read it as `$host.width()` (or `hostWidth` that is also used above when creating `$host`). Others may disagree, but to me it makes the relationship clear and unambiguous. The `75`, for instance, seems totally magical to me - is there anything wrong with using `1` (or even `0`)?