Unpack

A "pack" is not meant to be executable without ReproZip. It is supposed to be a simple, stable format that we don't need to worry about evolving.

Unpacking would recreate the actual runnable experiment on the target:
* [X] Install the necessary packages
* [ ] A directory containing the files, with a shell script that runs the experiment. The PATH and LD_LIBRARY_PATH variable would be adjusted for the program to run correctly (or we could ptrace again).
* [X] A directory we can chroot into.
* [ ] A virtual machine template for Vagrant (vagrantfile) with a provision script that would deploy the files there.

(for a VisTrails workflow to run the experiment, see #8)