I've added methods to to C4::Letters to manage the database table
message_queue. This will let us keep track of messages sent
via email, sms, and rss to patrons. That way, we can show the history,
deal with failures, and reconstruct an RSS feed when needed.

misc/cronjobs/overduenotics.pl has been added. It prepares advance notices
and item due notices and stages messages to be sent in the message_queue
table.

C4::Overdues::Getoverdues now takes two optional arguments to tell it how
old of overdues to fetch.

Also, a C4::Circualtion::getUpcomingDueIssues method was added that
advance_notices.pl uses.

misc/cronjobs/process_message_queue.pl has been added. It sends the email
or SMS messages out of the message queue.

The C4::SMS module didn't work at all, and it has been rebuilt to use
an external perl module from CPAN, SMS::Send.

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