feat: add app db schema extension, for CoreUser

need a way to map Wutta User to CORE Employee for auth purposes

tests/db/test_model.py

@@ -0,0 +1,16 @@
+# -*- coding: utf-8; -*-
+
+from wuttjamaican.testing import ConfigTestCase
+from wuttjamaican.db.model import User
+
+from wutta_corepos.db import model as mod
+
+
+class TestCoreUser(ConfigTestCase):
+
+    def test_str(self):
+        user = User(username='barney')
+        self.assertEqual(str(user), 'barney')
+
+        ext = mod.CoreUser(user=user, corepos_employee_number=42)
+        self.assertEqual(str(ext), 'barney')