Resolve "API job status could be more relevant"

Closes #349 (closed)

Following !186 (closed) note, the value of the status key is now the job.status.

Note that there is still a difference with rails implementation.

curl -H 'Authorization: Token token=...' -X GET https://allgo.inria.fr/api/v1/jobs/71771
{"71771":{"allgo.log":"https://allgo.inria.fr/datastore/500/206/c626b4afbd9f43d8b0b9a741a1bd986079f93f07/allgo.log"},"status":"done"}

curl -kH 'Authorization: Token token=...' -X GET https://localhost/api/v1/jobs/37
{"status": "SUCCESS", "37": {"allgo.log": "https://localhost/datastore/37/allgo.log"}}

Status in rails is 'done' on job success. Also note that job.status in rails is downcase, while not in django.
-> I'll make an issue for that. cf #356 (closed)