XD-1145. Add --date option to trigger

Please note I couldn't run the integration tests (part of this commit):

```
 $ cd /path/to/spring-xd
 $ ./gradlew clean :spring-xd-shell:test -Dtest.single=TriggerModulesTest
[...]
14/01/13 20:55:18 ERROR web.BasicErrorController: java.lang.NoClassDefFoundError: org/springframework/integration/core/MessageHandler
```

This error has been reproduced on @ericbottard's laptop.
Finally, it seems I already signed the CLA (for Spring Data, as far as I remember).