Fix: mail attachments should be placed in multipart/mixed MIME part.
Some formatting improvements in Mailer.buildMailBody
Improve variable names a bit to improve readability.
Add a comment documenting behavior of Exchange w/ mixed.

We added this code because some Exchange servers don't play nice without
a mixed multipart. This appears to be tied to some versions of the
Exchange server, not neccicarily Outlook or other associated clients.
Add a spec to cover the new multipart/mixed behavior.
Remove a println.
Merge pull request #1569 from lift/msf_email_attachments

Email attachments are now wrapped in a multipart/mixed segment to satisfy some versions of Exchange server that don't handle attachments correctly without this segment.