Took me a while to figure this one out, and I have a default installation of virtualenvwrapper -- this is a soft fix, just put an error message. But perhaps the fix should be to use the default value `~/.virtualenvs`.
* removes cd override by using chpwd_functions
* removes subshell call to which by using $+commands array and
c param expansion to find in PATH
* zsh love!
Using lazy loading for virtualenvwrapper gives a mariginal speed
improvement and doesn't stop workon_cd from working. It has the
undesired effect of forcing you to call certain virtualenv commands
twice before they work (only once per shell instantiation).
