Sign Mac OS X builds

[nokome]

The OS X binary release should be signed with an Apple developer key. As @rgbkrk said well in dat-ecosystem-archive/dat-desktop#120 it "relieves new users of additional points of friction with new tools".

Look into how similar projects do it:

• https://github.com/nteract/nteract/blob/master/RELEASING.md#macos
• dat-ecosystem-archive/dat-desktop#120

[nokome]

Quick update: application for a Stencila Developer ID in progress.