create DMGs for crosscompiled OSX packages

This applies to TBB as well (see gitian/descriptors/mac/gitian-bundle.yml: # XXX: DMG plz). Mozilla hasn't done this yet so anything we learn could also be helpful to them.

According to yet another stackexchange link it should be possible with something called genisoimage which has a git repository here:

https://github.com/hamstergene/libdmg-hfsplus

CCing people who I think might have an interest in the progress of this ticket.