don't require both email and username. closes #32

This change breaks backwards compatibility.

`createClient` no longer takes an `email` argument. Instead, the
`username` argument is used to authenticate and determine the
case correct username.

There is a special case if you leave out the `password` argument.
In this case, `username` is used to connect directly to the server,
and you may get kicked if the server is in online mode.
Release 0.7.0