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.

I had imagined "foo" and "bar" as the default values, and we would change them in our configuration. Oh, I guess there should also be another one for auth on/off as a feature flag. I'll add that.