support for auth plain (and maybe other auth modes)

[alexlehm]

For testing, it would be good to have different auth modes available and maybe also to be able to switch them on and off.

auth plain should be quite simple, other auth modes like cram-md5 could be faked with e.g. bouncycastle.

this would be helpful to test handling of login errors, e.g. if the client retries transmission on auth failure or not.