Add new package "xpwn" from @dborca's fork.

This really hase some nice utilities for coping with HFS+ and DMG
format. We're going to use it for extracting the xcode DMG, but it could
be used for more, even generating DMGs.

The reason I'm using the fork instead of the repository from
@planetbeing is thet the fork contains a few fixes, especially for
dealing with symlinks in HFS+ images.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>