packaging: generate DMG from app

Change-Id: I914b2fb19aeeb49437bc540871944f3bc77ba3bc
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>

generateDMG.json

+{
+  "title": "Ring",
+  "contents": [
+    { "x": 100, "y": 120, "type": "file", "path": "build-local/Ring.app"},
+    { "x": 500, "y": 120, "type": "link", "path": "/Applications" }
+  ],
+  "window": {
+    "size" : {"width" : 600, "height" : 300}
+  }
+}