Added device schedulement check
Checking whether there's a device scheduled in specified time

Signed-off-by: Carlos Oliveira <carlosol@member.fsf.org>
Signed-off-by: Áulus Diniz <aulus999@gmail.com>
Validating occurrences on schedule
Preventing from scheduling at the same period of time the same devices

Signed-off-by: Carlos Oliveira <carlosol@member.fsf.org>
Signed-off-by: Áulus Diniz <aulus999@gmail.com>