Store job configuration on job initialization.

Job configurations are stored in `spark.jobserver.filedao.rootdir`/config.data.
For every job, the jobId and jobConfig are stored: the configuration is written
as a concise json document.
Rename config.data to configs.data
Reorganize imports
Change saveJobConfig to use jobId instead of jobInfo

The code only used jobInfo.jobId anyway and this way the
JobDAO API makes it evident that one can use jobId to store
(and retrieve) the job config.
Sort imports
Remove item from TODO list.
Retrieve job configurations.

Job configurations are retrieved at startup from the configs.data
file and can be displayed using the following new route:

GET /jobs/<jobId>/config
Don't catch Exception so that it will bubble up
JobInfoActor should also handle the job configuration.

JobInfo now has a `jobConfig` field;
A job configuration is now stored along with its information;
Job configurations are managed separately from job information.
Get rid of extra space
Merge pull request #11 from fedragon/store-jobconfig

Thanks for the contribution