This change set adds the ability for karton instances to signal to downstream
consumers about the beginning and ending of task execution. Signaling is
performed by sending corresponding "status" tasks.

Task signaling is enabled using the following entry in the karton
configuration file:
```
[signaling]
status = 1
```

Task signaling is disabled by default.

* use `ConfigParser.getboolean`
* rename signaling status task identifiers