github-electron: Add app.dock.setIcon() API

https://github.com/atom/electron/blob/master/docs/api/app.md#appdockseticonimage-os-x

github-electron/github-electron-main-tests.ts

@@ -169,6 +169,7 @@ app.dock.setMenu(dockMenu);
 app.dock.setBadge('foo');
 var id = app.dock.bounce('informational');
 app.dock.cancelBounce(id);
+app.dock.setIcon('/path/to/icon.png');
 
 app.setUserTasks([
 	<Electron.Task>{

github-electron/github-electron.d.ts

@@ -1125,6 +1125,12 @@ declare module Electron {
 		 * Note: This API is only available on Mac.
 		 */
 		setMenu(menu: Menu): void;
+		/**
+		 * Sets the image associated with this dock icon.
+		 *
+		 * Note: This API is only available on Mac.
+		 */
+		setIcon(icon: NativeImage | string): void;
 	}
 
 	interface Task {