fix: add docs, tests; tweak some handler method signatures

tests/test_app.py

@@ -0,0 +1,16 @@
+# -*- coding: utf-8; -*-
+
+from wuttjamaican.testing import ConfigTestCase
+
+from wutta_corepos import app as mod
+from wutta_corepos.handler import CoreposHandler
+
+
+class TestWuttaCoreposAppProvider(ConfigTestCase):
+
+    def make_provider(self):
+        return mod.WuttaCoreposAppProvider(self.config)
+
+    def test_get_report_handler(self):
+        handler = self.app.get_corepos_handler()
+        self.assertIsInstance(handler, CoreposHandler)