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.
minor man page enhancement
changed to warning message when already in cache
enhanced --rebuild/--needed consistency with --devel
removed hybrid solver
removed AUR metadata check
minor config option reordering