This only works on Linux due to relying on traffic shaping (in
particular using the "tc" command).

Usage is as following:

    mount-slow-fs path/to/actual/repos/directory /mnt/mountpoint

This will mount the repositories directory at /mnt/mountpoint. Exiting
the script automatically unmounts the directory and removes the traffic
shaping rules.

Fixes #94, gitlab-com/operations#79