src/irclib.py: Delay sending SASL authstring until given green light

That means wait until receiving 'AUTHENTICATE +' from the server, which
means the mechanism is supported and we can proceed to send the
authstring.

It generally works anyway, but it's better to follow the standards, plus
old versions of elemental-ircd may crash if we don't do this.
Merge pull request #669 from nyuszika7h/patch/sasl-wait-auth

src/irclib.py: Delay sending SASL authstring until given green light