Fix Archlinux package creation

* When simply cloning from the repo, the folder name is PyBitmessage, not just pybitmessage.
* Building the `xxx-pkg.tar.xz` is the job of Archlinux's `makepkg`, not the job of upstream (ie bitmessage). You just need to build the `xxx.tar.gz` part

An archer still has to manually clone the repo to install it with the provided PKGBUILD. Is there any reason why you don't provide `xxx.tar.gz`-ified extracts of the code on each release ? (I also notice that you don't use tags, which could be really helpful with github). This would greatly help, because then all you'd need to distribute is the PKGBUILD along with an url pointing to the `xxx.tar.gz` (I think github hosts them automatically)