Support Google Play Games Service (#6147)

* Support Google Play Games Service

* clean up

* improve coverage

* more tests

src/Adapters/Auth/index.js

@@ -2,6 +2,7 @@ import loadAdapter from '../AdapterLoader';
 
 const apple = require('./apple');
 const gcenter = require('./gcenter');
+const gpgames = require('./gpgames');
 const facebook = require('./facebook');
 const facebookaccountkit = require('./facebookaccountkit');
 const instagram = require('./instagram');
@@ -35,6 +36,7 @@ const anonymous = {
 const providers = {
   apple,
   gcenter,
+  gpgames,
   facebook,
   facebookaccountkit,
   instagram,