Compression for joblogs

[wvengen]

After #28 logs can be stored on container storage.
Allow compression of logs, to reduce storage costs. This will also need to play well with #12.
May consider configuring compression (none / gzip / bzip2 / ...).

Ideally, the compression method can be changed and existing logs can still be found (one way would be to search through container storage, which could potentially be expensive; storing the final location as job metadata may be nicer).