Adapt the installer to the new buildsystem.

* prepare-packages.sh: Finds the speficied packages and copies
them into the build folder

Aligned the folder structure with that of all other tools:
  * Created Installer directory containing all files required for
    the .pkg
  * Copied all files from resources into Installer

Remove legacy files:
  * GPGTools-Autodownload.pkgproj
  * GPGTools-withFilesFor105.pkgproj
  * build-script.sh
  * build-script.sh.config
  * build-script.sh.config.nightlyDiff
  * fixInstaller.sh
Update README file with more accurate information.