Add tip calculator project structure and documentation

tests/test_calculator.py

@@ -0,0 +1,19 @@
+import unittest
+
+from tip_calculator.calculator import bill_per_person
+
+
+class BillPerPersonTests(unittest.TestCase):
+    def test_course_example(self):
+        self.assertEqual(bill_per_person(124.56, 12, 7), 19.93)
+
+    def test_tip_split_five_ways(self):
+        self.assertEqual(bill_per_person(150, 12, 5), 33.6)
+
+    def test_zero_people_raises(self):
+        with self.assertRaises(ValueError):
+            bill_per_person(100, 10, 0)
+
+
+if __name__ == "__main__":
+    unittest.main()