add AUR package completion to zsh.completion

Uses cower to list matching AUR packages.

You can disable AUR completion using the following zstyle:
```zsh
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.

Closes #269.

I've been trying to fix #236 as well but I still haven't found a nice way to do so.