Adds support for multiple keys existing in an encrypted DMG. When multiple keys are found it selects the first password key header.
Adds support for DMG sparseimage files. Previously the first chunk selected could be empty. Now chunk number one is chosen for DMG sparseimage files and a new heuristic is added to the DMG format plugin to look for 'Apple'.
Merge pull request #390 from mledford/dmg-fixes

Better DMG support