add zsh completion file

This adds a zsh completion file, as per Issue #152.

It is not perfect, but it works for most normal usage, with the exception of offering no completion for the pacman extensions -Q and -S. There's a zsh completion file for pacman available, but I'm unsure how to hook into it without copying the functions over.

Completion is also slightly incorrect when doing something like pacaur -si, which will assume that you want to install something, not want information, but pacaur --sync --info for example however will complete correctly (for --info).

All in all it should be an improvement to the current situation though, which is no completion at all :)