Added support for monitoring replication status using mysql::monit::slave

This functionality makes use of monit's capability to run arbitrary commands and
includes a (hopefully) POSIX-compliant script that verifies slave's status based
on the values of Slave_IO_Status, Slave_SQL_Status and Seconds_Behind_Master in
"SHOW SLAVE STATUS" output.

Signed-off-by: Samuli Seppänen <samuli.seppanen@gmail.com>