Serve favicon and ask it to be shown during OpenID

Fixes #90 

Before:

![screen shot 2013-06-25 at 12 26 45 pm](https://f.cloud.github.com/assets/24193/704246/b4ece68a-ddbc-11e2-923c-8868f4ede64b.png)

After:

![screen shot 2013-06-25 at 12 26 28 pm](https://f.cloud.github.com/assets/24193/704247/b8d2ee7a-ddbc-11e2-9254-d935622432d1.png)

Note: The Persona icon will not appear in local testing since http://127.0.0.1/favicon.ico is different for you than it is for Google's content proxy.

It is not possible to use favicons larger than 16x16. 

You can look at what favicons Google discovers for different domains by hitting their content proxy directly. Just change the `domain=` parameter:

https://s2.googleusercontent.com/s2/favicons?service=lso&domain=gmail.com

https://s2.googleusercontent.com/s2/favicons?service=lso&domain=microsoft.com

https://s2.googleusercontent.com/s2/favicons?service=lso&domain=github.com