fix the open command in linux using xdg-open
Update README.textile

Add upgrade command to docs. I canceled the prompt once and went hunting for this so I could upgrade.
virtualenvwrapper plugin cleanup

* 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!
virtualenv cleanup: replaces subshell with prompt expansion.

* :t parameter expansion returns the last portion of the path,
  equivalent to basename. I <3 zsh.
* adds comments for the VIRTUAL_ENV_DISABLE_PROMPT,
  used by virtual_env activate

See also:
  http://zsh.sourceforge.net/Doc/Release/Expansion.html#Parameter-Expansion