Enhancement: JobConfig could have a description

[neilstevenson]

A Jet job's JobConfig has a setName() method.

A setDescription(String s) method would be useful, to add additional information.

For example, on a job with a Kafka source, the description could be used to indicate which brokers are being used

[neilstevenson]

Part of the motivation here is to have some standard defined fields that describe the job, so that management center can display them sensibly.
For example, name is shown as is launch time.
Description would be the next obvious