Add Registry model

#623

test/models/registry_test.rb

@@ -0,0 +1,16 @@
+require 'test_helper'
+
+class RegistryTest < ActiveSupport::TestCase
+  def setup
+    @registry = Registry.send(:new)
+  end
+
+  def test_implements_singleton
+    assert_equal Registry.instance.object_id, Registry.instance.object_id
+  end
+
+  def test_vat_rate
+    Setting.registry_vat_prc = 0.2
+    assert 0.2, @registry.vat_rate
+  end
+end