Name .dmg files after the application bundle on Mac OS X

Use the name of the application bundle (input.outputPath) to give the
disc image a volume name instead of using the file name of the .dat
file ("out" is set to 'installer.dat' at construction time).

Change-Id: I0f53ca7fcafb83353f4fea61b62037f48281375e
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>