Support for authentication with username/password

[sheerun]

Please? I need it as heroku's memcached requires authentication.

[Jimbly]

Hi, I'm swamped with other things right now, so probably would not get to this for a while, but this is a pretty small library, so it might not be hard for you to add in. Pull requests are always welcome! It looks like this page probably details the protocol for connecting with authentication: https://github.com/couchbase/memcached/blob/master/docs/sasl.md