Add basic authentication middleware

@clytwynec @cgoldberg 

works with: http://foo:bar@localhost:5000
or by passing the credentials in the browser when prompted.

Credentials are configurable in the .ini file specified in the `paster serve config.ini` command.

These only get set in 'app_conf' if they are in the `.ini` file right?

If that's the case, how will we set them in heroku without checking in the ini file?