[DS-792] When mail sending is disabled, log the entire message instead.

Since this should only occur during debugging or testing, I thought it
expedient to just build the MimeMessage normally and then extract the
composed headers from it.  This is slightly inefficient, but greatly
simplified the code.

Attachments will not be logged, since they might be anything and of
any size.  Instead, a list of their names will be inserted below the
headers and above the message body.
Actually commit the attachment code.
Merge pull request #276 from mwoodiupui/DS-792

[DS-792] When 'mail.server.disabled = true' put text of email in log file