cylc shutdown|stop: fix using a datetime for shutdown

When using a datetime with `cylc shutdown` (e.g. `cylc shutdown 2013/08/14-15:00`) it was failing to work due to miscalling the datetime module:

```
2013/08/14 11:52:49 WARNING - 'module' object is not callable
2013/08/14 11:52:49 WARNING - Command failed: stop after clock time(2013/08/04-11:54)
```

This fixes the problem. I've also renamed date and time in the def concerned to sdate and stime just to clear up any confusion with the modules themselves.