Added an example using the identity API (#180)

* Added an example using the identity API

* Updated with review comment fixes

google-userinfo/background/userinfo.js

@@ -0,0 +1,22 @@
+/**
+Fetch the user's info, passing in the access token in the Authorization
+HTTP request header.
+*/
+function getUserInfo(accessToken) {
+  const requestURL = "https://www.googleapis.com/oauth2/v1/userinfo?alt=json";
+  const requestHeaders = new Headers();
+  requestHeaders.append('Authorization', 'Bearer ' + accessToken);
+  const driveRequest = new Request(requestURL, {
+    method: "GET",
+    headers: requestHeaders
+  });
+
+  return fetch(driveRequest).then((response) => {
+    if (response.status === 200) {
+      return response.json();
+    } else {
+      throw response.status;
+    }
+  });
+
+}