PKGBUILD: Download sources instead of including it into the AUR-tarball

The following changes the PKGBUILD and the Makefile:

PKGBUILD changes:
 * Download tagged downgrade version instead of included source files
 * This makes it necessary to enter the downloaded source

Makefile change:
 * Use "updpkgsums" instead of "sed + makepkg --geninteg >> PKGBUILD"
   This little program is included with pacman and is doing the same and a bit more.