add unit tests

tests/unit/test_cassettes.py

@@ -0,0 +1,12 @@
+import yaml
+from vcr.cassette import Cassette
+
+def test_cassette_load(tmpdir):
+    a_file = tmpdir.join('test_cassette.yml')
+    a_file.write(yaml.dumps([
+        {'request':'foo', 'response':'bar'}
+    ]))
+    a_cassette = Cassette.load(a_file)
+    assert a_cassette._requests
+    assert a_cassette._responses
+