expand the auth process to support multiple users

This patch allows the authentication of multiple users via a plain text database storing the username, salt, and PBKDF2 password hash of each user, which can be generated by a newly added tool called `spdyproxy-pwd.js`. Another newly added module called `auth.js` parses the database and performs authentication for the SPDY Proxy server, with caching of already authenticated users for optimal performance. The log output of the server is also tweaked to include the username of the initiator of each request.