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
Merge pull request #1878 from spazm/virtualenv-cleanup

virtualenv cleanup: replaces subshell with prompt expansion.
Merge pull request #1877 from spazm/virtualenvwrapper-cleanup

virtualenvwrapper plugin cleanup
Merge branch 'master' of https://github.com/robbyrussell/oh-my-zsh
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