Define schema for GCM push-notification settings

Remove unused property PushNotificationSettingSchema.platform.

Flatten GCM settings schema. There is no need to distinquish between
pushOptions and feedback, as there is only single HTTP channel shared
by both.

lib/models/application.js

@@ -23,11 +23,14 @@ var APNSSettingSchema = {
     }}
 };
 
+var GcmSettingsSchema = {
+  serverApiKey: String
+}
+
 // Push notification settings
 var PushNotificationSettingSchema = {
-    platform: {type: String, required: true}, // apns, gcm, mpns
-    // configuration: {type: Object} // platform-specific configurations
-    apns: APNSSettingSchema
+    apns: APNSSettingSchema,
+    gcm: GcmSettingsSchema
 };
 
 /**