Add autoenv plugin, which adopts using Kenneth Reitz's autoenv.

Hi, 

I've added a plugin for using [autoenv](https://github.com/kennethreitz/autoenv). When cd'ed into a directory, autoenv checks whether there is an .env file in this directory; if there is one it executes the contents of this .env file.

This commit changes the overridden cd () in lib/directories.zsh. The plugin assumes autoenv is installed under ~/.autoenv a.k.a autoenv is git checked-out to .autoenv. I'm open to any suggestions about that.

The use_env function in the plugin is used to auto-activate virtualenv and needs virtualenvwrapper. The code is taken from https://github.com/kennethreitz/autoenv/wiki/Cookbook

For the autoenv users, I think this plugin would help much. I'd be pleased if you merge the code into master branch. I'd also appreciate any comments on how to improve the code.

Peace!

-Serdar