[WIP] Backup compression option

As requested [here](http://feedback.gitlab.com/forums/176466-general/suggestions/5667052-allow-compression-of-backups-using-bzip2-gzip-etc), this PR add a compression option for backup.

TODO list:

- [x] Compression settings on gitlab.yml
- [ ] Implement compression commands on backup create
- [ ] The restore method should guess compression type and extract it to avoid BC break
- [ ] Improve clean old backups method to fetch compressed packs
- [ ] Check backup upload method to fetch compressed packs
- [ ] Check for backup test and update it
- [ ] Check if docs could be updated
- [ ] Give some benchmark with dev env
- [ ] Update CHANGELOG

This feature would be useful to save some space especially for big Gitlab platforms.

Tell me if I missed some check on todo list!
Prefer single-quoted strings when you don't need string interpolation or special symbols.