Add GitHubElectron.App.getAppPath()

Electron v0.30.0 introduced it.

https://github.com/atom/electron/releases/tag/v0.30.0

github-electron/github-electron.d.ts

@@ -855,6 +855,10 @@ declare module GitHubElectron {
 		 * cleanup code will not run.
 		 */
 		terminate(): void;
+		/**
+		 * Returns the current application directory.
+		 */
+		getAppPath(): string;
 		/**
 		 * @param name One of: home, appData, userData, cache, userCache, temp, userDesktop, exe, module
 		 * @returns The path to a special directory or file associated with name.