Add support for DMG files

[ci-macpaw]

Originally reported on Google Code with ID 680

samuelesandeen:

I assume the summary is simple enough. Support for iso and other disk image formats
exists. I would like to avoid having such things mounted as disks and instead just
extract their contents.

[ci-macpaw]

mikerich88:

DMGs have so many different combinations of compression and encryption and filesystems,
and there is already a GPLv3 project for them: http://www.catacombae.org/dmgextractor/

[ci-macpaw]

adamjay:

If there is anything I can do to help provide encouragement on this, as well as .sparsebundle
support please let me know!  Often I need to extract a file or two from an encrypted
.sparsebundle and it would be nice if I could do that from an iOS device on the go!

[ci-macpaw]

paracelsus:

This is terribly complicated, but it would be a good feature. I'll need to be very bored
some day to start working on it, though.