Add the Supermarket Beta banner.

:fork_and_knife: The Supermarket Beta banner is a global drop down that contains information
surrounding the Supermarket Beta. This banner will only be around until the Beta
ends.

- Trello card for this: https://trello.com/c/G88oZ2KE
- Invision mock-ups for this:
  - [collapsed banner](https://projects.invisionapp.com/share/WXTGY83T#/screens/22551982)
  - [expanded banner](https://projects.invisionapp.com/share/WXTGY83T#/screens/22551980)


The content contains two key pieces that are displayed side-by-side:

- what users should expect
- how users can get involved

The banner features a slide toggle when pressed and emulates the look and feel
of buttons throughout the application. On mobile, the two pieces of content
collapse into one stacked column.

![beta-banner-toggle](https://cloud.githubusercontent.com/assets/928367/2907181/78daa96a-d619-11e3-8e70-8d8be775ed9c.gif)

I would totally appreciate feedback on the design, implementation and copy!