[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
- [x] Implement compression commands on backup create
- [x] The restore method should guess compression type and extract it to avoid BC break
- [x] Improve clean old backups method to fetch compressed packs
- [x] Check backup upload method to fetch compressed packs
- [ ] Check for backup test and update it
- [x] Check if docs could be updated (upgrade)
- [x] Give some benchmark with dev env
- [x] Update CHANGELOG

This feature would be useful to save some space especially for big Gitlab platforms.

Some benchmark, done on my workstation with default dev env and data:

Extension | Time | Size
--- | --- | ---
.tar | 0m7.746s | 101M
.tar.gz | 0m10.158s | 96M
.tar.bz2 | 0m20.678s | 96M
.tar.xz | 0m35.414s | 80M

This benchmark use default compression level.

Tell me if I missed some check on todo list!
Same problem as here: https://github.com/gitlabhq/gitlabhq/pull/7962#discussion_r18450832