Add additional unittest assertion to test_SMTPAutoresponder_getMailTo_UnsupportedDomain().

If anything in this list ends up in the list of permitted client email
addresses, we should know about it, and also fail the test.
Fix bug with weird '550 Sender not permitted' SMTP responses.

When raising a `twisted.mail.smtp.SMTPBadSender`, we were giving it the
origin's domain name as argument, which ended up with weird messages
like '550 Cannot receive from specified address <example.com@localhost>:
Sender not acceptable' being sent out. Apparently, SMTPBadSender want
the whole origin object, not just the domain.

 * CHANGE raising of SMTPBadSender in
   b.e.server.SMTPIncomingDelivery.validateFrom() to use the whole
   origin object, not just the domain, as its argument.
Make docstring for b.e.server.addServer clearer.

Since there are technically two SMTP servers, be clearer that this
function sets up the one which listens for incoming emails.
Add docstring note that b.p.a.normalizeEmail can raise UnsupportedDomain.
Check for whitelisted email addresses before any more complicated parsing.

 * CHANGE getMailTo() and runChecks() in
   b.e.autoresponder.SMTPAutoresponder to check for whitelisted
   addresses first, before any of the other checks.
Add whitelist attribute to bridgedb.Dist.EmailBasedDistributor.

 * ADD bridgedb.Dist.EmailBasedDistributor.whitelist attribute which
   should hold the contents of the EMAIL_WHITELIST setting in
   bridgedb.conf.

 * CHANGE createBridgeRings() function in bridgedb.Main module to use
   the new email distributor whitelist attribute, and pass it the
   settings from the parsed config file.
Remove superfluous normalizeEmail() call.

 * REMOVE second call to normalizeEmail() in
   EmailBasedDistributor.getBridgesForEmail(), and add note about where the
   checks are currently called.

 * ADD some extra logging when we receive imcoming requests from email
   addresses in the EMAIL_WHITELIST.

 * TODO check that incoming emails from addresses in EMAIL_WHITELIST have
   valid signatures made by the key with the corresponding GnuPG key
   fingerprint (as in EMAIL_WHITELIST, which is a mapping of addresses to
   fingerprints).