Switch default filesystem from ext3 to ext4

It should be safe to default to ext4 nowadays with Debian/wheezy
being the stable Debian release.
Provide --debug option for very verbose execution

This effectively executes grml-debootstrap under 'set -x'
(including the chroot script), so it's easy to generate an
execution log via e.g.:

  grml-debootstrap ... --debug --force 2>&1 | tee -a grml-deboootstrap.log

and providing it to grml-debootstrap developers for further
investigation.
Support execution of --grub when installing to target directory

Thanks: Michael Renner <michael.renner@amd.co.at> for initial bug report and sponsoring my development time