Add zshenv file

From the [Zsh manual](http://zsh.sourceforge.net/Intro/intro_3.html):

> '.zshenv' is sourced on all invocations of the shell, unless the -f option is
> set. It should contain commands to set the command search path, plus other
> important environment variables. `.zshenv' should not contain commands that
> produce output or assume the shell is attached to a tty.
>
> `.zshrc' is sourced in interactive shells. It should contain commands to set up aliases,
> functions, options, key bindings, etc.

So why is this important? [Alfred](http://www.alfredapp.com/) workflows run in non-interactive shells. When the `$PATH` is set or `rbenv` is initialized in `zshrc` instead of `zshenv`, those workflows will not use the correct Ruby version and might not have access to certain bin files, such as those from `$HOME/.bin/` or Homebrew.

I've created a [simple workflow](http://cl.ly/450K3t3b423H) to demonstrate the problem if you'd like to see for yourselves. Install it and type `rbenvtest` into Alfred.

Before:
![screen shot 2014-07-27 at 11 04 55 pm](https://cloud.githubusercontent.com/assets/16124/3716191/01b273f0-1605-11e4-8dcf-87eb30211f40.png)
After:
![screen shot 2014-07-27 at 11 06 35 pm](https://cloud.githubusercontent.com/assets/16124/3716193/106aae58-1605-11e4-98fd-52ccf8bdc064.png)