Zope's MailHost overhauled

Zope's MailHost got a new engine under the hood. The sending and delivery part of MailHost is now based on zope.sendmail. This brings us several advantages:

• integration with the Zope transaction system (no longer dupe emails in case of conflict errors)
• TLS/SSL support (if available (requires Python to be compiled with SSL support))
• optional asynchronous mail delivery using a mail queue and a queue processing thread
  

With this implementation we should be able to get rid of almost all other MailHost flavours since Zope supports their functionality out-of-the-box.