Allow password login

This is already mentioned in the README, but I thought I'd best create an issue for it.

As I understand it, this requires:
a) a way of specifying the usernames+passwords (much like `addPublicKey` does)
b) a way of checking these in `client.on('authentication', ...)`

I will have a go at this and see where I get. If I've missed something from my plan-of-action above, let me know.