add schedule-change to sw

allow changing a service schedule by implementing commands:

```sh
sw shift SERVICE SHIFTNAME START STOP SETUP TAKEDOWN
```

- add or replace shift `SHIFTNAME` of service `SERVICE` with given `START` and `STOP` times, and `SETUP` and `TAKEDOWN` scripts

and

```sh
sw delshift SERVICE SHIFTNAME
```

-  delete the shift named `SHIFTNAME` from service `SERVICE`  