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!