Add virtualenvwrapper hook files

- hook files are stored in virtualenvwrapper.
- tools/bootstrap.sh sets up symlinks
- postactivate and postdeactivate call zsh rehash
- TODO: implement uninstall
Add virtualenv to RPROMPT
postmkvirtualenv: Install basic packages in all virtualenvs
Set PIP_DOWNLOAD_CACHE
No longer make --no-ff the default merge
Merge branch 'master' into mac