replace gzip with pigz to make backups a lot faster

A user on IRC complained yesterday that we use gzip in our backup task.
He complained because gzip only uses 1 core.
To speed up the backup process on multicore machines I propose to use pigz instead of gzip.
Info about pigz: http://www.zlib.net/pigz/

It is a drop in replacement for gzip and it decreases the time needed for the backup creation a lot!
I benchmarked it a bit with my eclipse installation because i didn't have a gitlab backup lying around and got the following results:
(same commands that we use in gitlabs backup script)

Gzip:

executed command:
time tar -C ./eclipse/ -cf - . | gzip -c -1 >backup_gzip.tar

results:

real	0m17.750s
user	0m17.255s
sys	0m1.145s

Pigz:

executed command:
time tar -C ./eclipse/ -cf - . | pigz -c -1 >backup_pigz.tar

results:

real	0m5.635s
user	0m16.333s
sys	0m0.801s

Pros:

As you can see this speeds up the time considerably.

Cons:

The downside is, that we either have to bundle pigz in the omnibus install or require it as a dependency.
It is in the repos of most distributions, but not installed by default.

/cc @yorickpeterse @marin @dblessing