Arch Build System Packages

![archlinux-logo-dark](https://f.cloud.github.com/assets/687638/334269/214d9fb6-9c6f-11e2-9812-f91c609604c3.png)

I've created 2 PKGBUILD files for Arch Linux users.

* `wp-cli` is the stable release, based on the latest tagged version.
* `wp-cli-git` is the rolling development release, based on the repository.

Any input, testing, or contributions would be appreciated. This is my first run at creating packages.

I'll submit it to the AUR after further testing. I'm getting one error building the package regarding fakeroot, which doesn't halt the build, and never should need to be used anyway. Anyone installing packages with pacman will have root or sudo access.