Added pew and virtualenv-clone
Changed virtualenv-clone to use the version from pypi (the author will keep updating it, so I don't need my fork on github anymore)
Tried to made the tests run with tox: updating tox (and the dependencies: virtualenv and py) let the installation finish, but its tests still fail
Merge branch 'pew' of git://github.com/berdario/nixpkgs

Added Pew (and misc. related changes)

Conflicts:
	pkgs/top-level/python-packages.nix