Make it easier to define timer units for services

Systemd services now have a startAt attribute.  If set, NixOS will
automatically emit a timer unit that causes the service to start at
the specified time.  For example:

  systemd.services.foo =
    { script = "... bla bla ...";
      startAt = "02:15";
    };

causes the given script to be started at 02:15 every day.