virtualenv cloning script.

A script for cloning a non-relocatable virtualenv.

Virtualenv provides a way to make virtualenv's relocatable which could then be
copied as we wanted. However making a virtualenv relocatable this way breaks
the no-site-packages isolation of the virtualenv as well as other aspects that
come with relative paths and '/usr/bin/env' shebangs that may be undesirable.

Also, the .pth and .egg-link rewriting doesn't seem to work as intended. This
attempts to overcome these issues and provide a way to easily clone an
existing virtualenv.

It performs the following:

- copies sys.argv[1] dir to sys.argv[2]
- updates the hardcoded VIRTUAL_ENV variable in the activate script to the
  new repo location. (--relocatable doesn't touch this)
- updates the shebangs of the various scripts in bin to the new python if
  they pointed to the old python. (version numbering is retained.)

    it can also change '/usr/bin/env python' shebangs to be absolute too,
    though this functionality is not exposed at present.

- checks sys.path of the cloned virtualenv and if any of the paths are from
  the old environment it finds any .pth or .egg-link files within sys.path
  located in the new environment and makes sure any absolute paths to the
  old environment are updated to the new environment.

- finally it double checks sys.path again and will fail if there are still
  paths from the old environment present.

NOTE: This script requires Python >= 2.5