virtualenvwrapper 1.4, now with .pth management

Version 1.4 of virtualenvwrapper includes a .pth file management function contributed by James Bennett and Jannis Leidel. The new add2virtualenv function makes it easy to share code between virtual environments without installing it in the system site-packages directory, by adding directories to a .pth file in the virtualenv.

Thanks, James and Jannis!