Breaking news alert + a/b test

Updates the alert style to slide up the alert after 5 seconds and pin it at the bottom of the screen, like this:

![screen shot 2014-12-03 at 12 49 42](https://cloud.githubusercontent.com/assets/867233/5280536/773ba192-7aea-11e4-8d40-78bc457c72bc.png)

![screen shot 2014-12-03 at 15 50 27](https://cloud.githubusercontent.com/assets/867233/5283158/d86f4356-7b03-11e4-9517-0aa58aab9169.png)

Also puts the display behind a 50/50 show/don't show A/B test.

To do:
- adding manual tracking to record the alert being shown as well as just clicked

cc @paulrobertlloyd @rich-nguyen @stephanfowler 
I was talking about this with Austin this morning and we will introduce some z-index context variables so we will have clear and maintainable z-index across the code base.