Added bare-minimum support for creating apple's DMG files.

* Added method to take a mappings and generate a DMG file.
* added package-osx-dmg method to universal (only works on mac).

Note on dmg:  It's just a generic way to create disks and mount applications.
I'm not positive it makes sense to do so outside of app bundles, but we'll at least
provide the hook, so OSX users can figure out what they want.
Merge pull request #13 from sbt/wip/create-dmg-files

Added bare-minimum support for creating apple's DMG files.