Store job configuration on initialization

I implemented some changes to store jobs' configurations, as mentioned in the following item from `README.md`'s TODO list:

> Store the config used to launch a job.

Job configurations are stored upon job initialization, in `${spark.jobserver.filedao.rootdir}/configs.data`.
For every job, the jobId and jobConfig are stored and the configuration is stored as a _concise_ json document (as in `com.typesafe.config.ConfigRenderOptions.concise()`).