Build, sign, notarise, & upload on OS X (#687)

* sign & notarise darwin package

* upload bundle as github release

* allow unsigned build and full build with release

* deref darwin bundle symlinks only when building on win32

Windows [mangles symlinks](https://github.com/electron/electron-packager/issues/71). Currently we work around this by placing several copies of the frameworks in OS X app bundles (see 1e5aa3867d). However:

- This is invalid: the framework toplevel must [only contain symlinks](https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/FrameworkAnatomy.html). `codesign` [refuses to sign](https://stackoverflow.com/questions/25969946/osx-10-9-5-code-signing-v2-signing-a-framework-with-bundle-format-is-ambiguou) this invalid structure.

- It seriously bloats the bundle.

Since there's no fix for the Windows misbehaviour, keep the workaround, but only when cross-building on win32 for darwin; and log a warning.

gulp/buildutils.js

@@ -25,6 +25,14 @@ module.exports = {
             });
     },
 
+    getTag() {
+        try {
+            return execSync("git describe --tag --exact-match").toString("ascii");
+        } catch (e) {
+            throw new Error('Current git HEAD is not a version tag');
+        }
+    },
+
     getVersion() {
         return trim(fs.readFileSync(path.join(__dirname, "..", "version")).toString());
     },