Introduced StockChange. Implemented first part of stockit-logic with updating article's quantity.

test/unit/stock_change_test.rb

@@ -0,0 +1,8 @@
+require 'test_helper'
+
+class StockChangeTest < ActiveSupport::TestCase
+  # Replace this with your real tests.
+  test "the truth" do
+    assert true
+  end
+end