adding Basic Auth Handler

I'd like to have web-platform-tests/webdriver start using this server instead:

https://github.com/lukeis/web-platform-tests/commit/0128116fdc00283ff3e4f31d4865adeb820dfb29

this change was required to have a way to configure how to handle http auth requests. Let me know if there's a more pythonic way to deal with the credentials.