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The default timeout seems to be 60 seconds which is much too long.

Fixes #130

- Use nbxmpp instead of python-nbxmpp
- Unify indentation

RFC 6120 states that ending the stream after successful auth is not
allowed because the stream is considererd ended. Sending </stream:stream>
would run into a parser error. So we restart the stream and end it afterwards.

VALIDATE_ALL is the default according to the docs

Fixes #126