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 13 59 49](https://cloud.githubusercontent.com/assets/867233/5281357/45ef2848-7af4-11e4-950e-8b5c1de3e81f.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 
means 50% see the alert, 50% won't. happy to reduce but not sure what the risk of 100% would be?