Add favicon / logo

I'm having trouble telling which of my tabs are Passport documentation, so I made a logo suitable for a favicon:

![passportjs](https://f.cloud.github.com/assets/572196/195581/9fe76ada-7ff7-11e2-9c42-7d8649d3e2b2.png)

https://github.com/stuartpb/passport-logo

Feel free to integrate it into your own repos however you'd like; I've released it under a CC-BY-SA license.