bug 2274 [1/5] database changes (DB rev 097) to add email address to message_queue...

bug 2274 [1/5] database changes (DB rev 097) to add email address to message_queue table

I added three fields to the message_queue table:
 - to_address
 - from_address
 - content_type: necessary to hold email header for attachments
I changed one field in message_queue:
 - borrowernumber can now be NULL for messages sent to the admin

These can be used to override the default addresses used. By default, emails are sent to
the borrowers.email address and sent from the KohaAdminEmailAddress syspref address. These
values are still used if the to_address or from_address values are NULL. These changes
are necessary to let overdue_notices.pl be backwards compatible.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>