Check if the virtualenv has a project directory when you invoke the workon command

If a virtualenv has an associated project directory, use this directory as the current working directory (cwd) in the sub-shell.  
Right now, if you make a new project, the cwd is changed to the project directory, but if you activate the virtualenv with 'pew workon', the cwd is not changed. IMHO, this behavior doesn't make much sense.

By the way, virtualenwrapper changes your cwd to the virtualenv project directory.