Update tools and instructions for building a signed Mac DMG

This updates the bundle to include the mime.cache and a library that
somehow isn't picked up by the bundle tool.

It also updates the README on how all this is supposed to work and puts
some of the automation into the existing shell script.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Merge branch 'mac-dmg'