Create and add derbynet-timer.service to derbynet-extras

[dougfultz]

To simplify setup of the startup of the DerbyNet timer. Please create and add /usr/share/derbynet/conf/derbynet-timer.service to the derbynet-extras package with the following contents:

[Unit]
Description=DerbyNet Timer
After=multi-user.target

[Service]
ExecStart=/usr/share/derbynet/scripts/derby-timer.sh
Type=simple

[Install]
WantedBy=multi-user.target

This will simplify setup by allowing a user to edit /etc/derbynet.conf with their desired configuration, then enable the automated start up with:

$ sudo systemctl enable /usr/share/derbynet/conf/derbynet-timer.service