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.