Add /{jobid}/directories route

[sverhoeven]

Fixes #55

TODO

• [x] think harder about return shape. Looks a lot like os.DirEntry which is ok.
• [x] add prefix, so you can list files in a sub dir only.
• [x] make async
• [x] add tests
• [x] add review instructions

To test:

1. Setup bartender with word count application
2. Follow https://github.com/i-VRESSE/bartender#word-count-example instructions
3. Try out the GET /api/job/{jobid}/directories route
   • It should return listing with files named: README.md, stdout.txt, stderr.txt, returncode and meta .
4. Try out the GET /api/job/{jobid}/directories/{path}
   • As word count application does not have sub directories you should make one by running mkdir /tmp/jobs/1/somedir && /tmp/jobs/1/somedir/somefile
   • Use somedir as value for path parameter.
   • Should return somefile as file.