Set CSS top for the placeholder contentContainer

# Problem
The content container would disappear off the viewport if the top was not set
# Solution
Set CSS top for the placeholder contentContainer
# How to test
- Checkout this branch
- `./init.sh && grunt qa` tests should pass
- your browser should open localhost:9000
- click the scrollList demo
- inspect the DOM and find the element with class `scrollList-contentContainer`
- That element should have a top: 0px CSS property

![image](https://cloud.githubusercontent.com/assets/6053699/3807047/79d9b2f0-1c5c-11e4-9fdb-4b4c26f73405.png)

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