add AUR package completion to zsh.completion

Uses cower to list matching AUR packages.
You can disable AUR completion using the following zstyle:

    zstyle ':completion:*:pacaur:*' remote-access false

The name `remote-access` mimics other completion zstyles like _cvs and _scp
that use this for deciding whether to complete remote files or not.
Merge pull request #272 from lucy/zsh-aur-completion

add AUR package completion to zsh.completion