Rename b.e.server.MailContext to b.e.server.MailServerContext.
Remove local hostaddr domain pass for SMTP checks.

 * CHANGE b.e.server.MailDelivery class to check incoming DNS names,
   never addresses.
Refactor b.e.server.MailMessage class.

 * ADD attributes so that the information about the incoming SMTP
   message is still accessible to the autoresponder while we're creating
   the reply.
 * FIXUP docstrings.
Rename b.e.server.MailDelivery → b.e.server.SMTPIncomingDelivery.

 * ADD more documentation to b.e.server.SMTPIncomingDelivery.
Rewrite 'Received:' header generation function to make more standard headers.
Refactor b.e.server.MailFactory.

 * CHANGE b.e.server.MailFactory into two separate classes,
   b.e.server.SMTPIncomingDeliveryFactory and
   b.e.server.SMTPIncomingServerFactory. This will enable us to be able
   to keep state for multiple emails coming in over a single connection,
   if necessary.
 * CHANGE b.e.server.addServer to use the new classes.
Add unittests for bridgedb.email.dkim module.