Add git_get_root function which returns root of current repo
Use  in Virtualenvwrapper plugin
Fixes venvwrapper deactivate when switching git repos
Use local variables.
Use chpwd hook instead of overriding cd command
Merge branch 'virtualenvwrapper_improv' of git://www.github.com/barraponto/oh-my-zsh

Conflicts:
	lib/git.zsh
	plugins/virtualenvwrapper/virtualenvwrapper.plugin.zsh