Allow fallback from CRAM-MD5 in t.m.smtp

[philmayers]

As documented in this Thunderbird ticket:

 https://bugzilla.mozilla.org/show_bug.cgi?id=311657

...some SMTP servers present CRAM-MD5 in the EHLO banner, but cannot successfully complete a CRAM-MD5 authentication for some/all users (perhaps because the relevant secrets are absent server-side).

Twisted will currently fail with these servers; it always tries CRAM-MD5 first, and does not fall back. See:

 http://twistedmatrix.com/pipermail/twisted-python/2012-July/025876.html

There are obviously security considerations here - it might be a man-in-the-middle attack.

More generally, there is no way to influence the set and behaviour of auth methods used by the Twisted SMTP code; the ESMTPSender class uses a private method, _registerAuthenticators, to initialise the list from init