Add "How does this work?" page

:warning: Still a work in progress, and not ready to be merged :warning: 

This adds a basic "How does this work?" page, which explains how the app works, and which OAuth scopes are requested. This page will be shown to people before they have authorised the application.

Here's how it looks:

![screen shot 2014-08-09 at 17 17 24](https://cloud.githubusercontent.com/assets/65057/3866576/56967f7e-1fd8-11e4-85ca-aa2a902d10d0.png)