changed auth process in stream.js + digestmd5

Changed a bit the onAuth + authenticate process in /lib/c2s/stream.js

Changed PLAIN authentication and added digestmd5 to follow the new standard. in this way even if we want to add a new auth mechanism we don't have to change the stream.js

If you want to use digestmd5 you need to listen on to the event "authenticate-digestmd5" and pass the password when required then the digestmd5 will check.

example:
https://gist.github.com/DevAlien/dd91d38c01748aeba453