add option --auth-string to simply basic authentication.

Add BasicAuth to chef-zero.

If given option `-auth-string STRING` or initialize ChefZero::Server with `:auth_string`  `server = ChefZero::Server.new(:auth_string => 'foobar')`, then chef_server_url will be provided as `'http://foobar@localhost:8889'`

UseCase:

1. We run chef-client local-mode on a shared-hosting server with recipe which spends long time.
2. The server listens local port during chef-client running.
3. Maybe, shared-hosting users could retrieve my cookbooks or other resources via http://localhost:8889.