Add the option to use a hosted domain with google's oauth

The hd option is used to restrict which email domain that are allowed to
log in to your app.

Starting from this commit you can pass `hostedDomain: 'example.com'` to
only allow emails from the domain `example.com`.

packages/google/google_client.js

@@ -44,6 +44,10 @@ Google.requestCredential = function (options, credentialRequestCompleteCallback)
         '&access_type=' + accessType +
         '&approval_prompt=' + approvalPrompt;
 
+  if (options.hostedDomain) {
+    loginUrl += '&hd=' + encodeURIComponent(options.hostedDomain);
+  }
+
   Oauth.initiateLogin(credentialToken,
                       loginUrl,
                       credentialRequestCompleteCallback,