Change rename.vim to Tim Pope's vim-eunuch

Does what rename.vim does (`:Move` or `:Rename`) **plus**:

* Adds `:Unlink` or `:Remove` to delete the current buffer + file
* Adds `:Mkdir` (with no argument, create the current file's containing
  directory)
* Adds `:SudoWrite` if you forget to edit a file as root
* Automatically chmods a file to `+x` if it starts with `#!`
* Tim Pope! :heart:
Force vertical diffs

Fugitive was updated to switch to horizontal diffs on narrow screens.
Everyone I've seen experience this behavior finds it disorienting. This
setting forces a vertical diff without users having to use different
shortcuts to enter diff mode.
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.

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.
Remove old doc directory

Resolves #276