Adds 'unpack installpkgs' command

This installs the packages required by a pack, and reports differing
versions. For instance:

    sudo -E reprozip unpack installpkgs experiment.rpz
Drafts 'unpack chroot'
Oops! Python 3 compatibility

A print statement made its way to the commit!
Merge branch 'master' into 'unpack'
Creates directories used as working directories
Parses output from ldd to copy /bin/sh + deps
Fixes for 'unpack chroot'
Adds 'unpack vagrant'
Splits the project in two packages
Merge branch 'unpack-vagrant' into 'unpack'

Conflicts:
	reprounzip/reprounzip/unpack.py
Style corrections

My indentation differs a bit from PEP8 (because I indent continuation
lines twice under most circumstances, triggering E126).
Merge branch 'master' into 'unpack'

Conflicts:
	.travis.yml
Merge branch 'master' into 'unpack'

Adds DATA/ in the packed tar.
Allows 'reprounzip graph' to read a pack
Fixes classifiers for reprounzip
Adds more system info to 'runs' section

This is needed to correctly select a Vagrant box.
Merge branch 'master' into 'unpack'
Tries to get missing package files from host
Checks that paths in tar archive are sane
Makes shell_escape() less aggressive