More informative (or possibly custom) messages

[knowtheory]

current log messages in the server log look like:

2015-02-05-22:29:16: Job 2170513 WorkUnit 7748950 large_document_import succeeded in 0.13942971
2015-02-05-22:29:16: Job #2170513 (large_document_import) succeeded.

It would be nice if it were clear what kind of WorkUnit each unit is (whether it's a split, process or merge for example), or even go so far as to set up a system where completed/failed work units, when posted back to the central server include a customizable message in the JSON data blob it each worker sends.