Update mac packaging tool to create code-signed .app packages

It took quite a lot longer than I expected, but I can now produce code-signed .app bundles.

The long and twisty path was:
+ Get a code signing certificate, owned by Bitcoin Foundation, Inc.
   (...several boring subtasks involved...)
+ Upgrade my development machine to OSX 10.8
   (code signing not working properly on OSX 10.6)
+ Fix macdeployqtplus so it works on 10.8
   (python appscript package not being maintained any more, switch to osascript)

I'll create a code-signed 0.7.2 .app and will recruit some 10.8 users with gatekeeper turned on to test.