Add screenshot to Authentication page

![2014-10-09-154503_1169x826_scrot](https://cloud.githubusercontent.com/assets/1651943/4582919/f77a837e-4fec-11e4-8e1c-60d0be3bd9aa.png)

This is just a screenshot from my local, so no sensitive data. It shows where the API token can be found. How should we host this? Can we make it a static file in mkdocs?