add ```created_by``` column to the job table

[hughjfchen]

Shall we add a created_by column to the job table so that we can track who submits the job and list all jobs submitted by some one?

[hughjfchen]

Think twice. Add a created_by column to the job table may be not a good idea considering that this is a library and only focus on job dispatching and running. I'd revise my database design and normalize the schema and create a new table to represent the relationship between the job and user. That way, we can keep the job table focusing on job and clean.