Add unit testing via pytest (#987)

* Add pytest to project

Includes a couple sample tests, a note in CONTRIBUTING, and inclusion in the makefile

* Include pytest in Travis build

tests/test_time.py

@@ -0,0 +1,18 @@
+import datetime
+from jrnl import time
+
+
+def test_default_hour_is_added():
+    assert time.parse(
+        "2020-06-20", inclusive=False, default_hour=9, default_minute=0, bracketed=False
+    ) == datetime.datetime(2020, 6, 20, 9)
+
+
+def test_default_minute_is_added():
+    assert time.parse(
+        "2020-06-20",
+        inclusive=False,
+        default_hour=0,
+        default_minute=30,
+        bracketed=False,
+    ) == datetime.datetime(2020, 6, 20, 0, 30)