Add About page

:warning: Still a work in progress, and not ready to be merged :warning: 

This adds a basic "About" page, which explains how the app works, and which OAuth scopes are requested.

Here's the initial mockup:

![screen shot 2014-08-08 at 14 55 28](https://cloud.githubusercontent.com/assets/65057/3856711/5211240a-1efb-11e4-8b74-1f305b84e60d.png)