Tyr: add an api who return jobs

This commit add two new fields on the job table, so the migration must
be run.
example:

    http://localhost/v0/jobs/

it's possible to filter by instance by adding the instance name after
the url:

    http://localhost/v0/jobs/foo
Tyr: always return a pretty json
Merge pull request #467 from kinnou02/tyr_jobs_api

Tyr: add an api who return jobs