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 create the actual runnable experiment on the target, either:
* A directory containing the files, with a shell script that runs the experiment. Could also try to install the needed packages. The PATH and LD_LIBRARY_PATH variable would be adjusted for the program to run correctly (or we could ptrace again).
* A virtual machine template for Vagrant (vagrantfile) with a provision script that would deploy the files there.
* A VisTrails workflow to run the experiment (do we need to set a vistrail variable here, with paths?)