Added model for offers table

refs https://github.com/TryGhost/Team/issues/1088

- adds basic model definition for `offers` table

core/server/models/offer.js

@@ -0,0 +1,13 @@
+const ghostBookshelf = require('./base');
+
+const Offer = ghostBookshelf.Model.extend({
+    tableName: 'offers',
+
+    product() {
+        return this.belongsTo('Product', 'product_id', 'id');
+    }
+});
+
+module.exports = {
+    Offer: ghostBookshelf.model('Offer', Offer)
+};