bring back the files module, but do serialization in cassette

vcr/files.py

@@ -0,0 +1,19 @@
+import os
+import yaml
+
+# Use the libYAML versions if possible
+try:
+    from yaml import CLoader as Loader, CDumper as Dumper
+except ImportError:
+    from yaml import Loader, Dumper
+
+def load_cassette(cassette_path):
+    return yaml.load(open(cassette_path), Loader=Loader)
+
+def save_cassette(cassette_path, data):
+    #TODO: safe overwrite using tmpfile
+    dirname, filename = os.path.split(cassette_path)
+    if not os.path.exists(dirname):
+        os.makedirs(dirname)
+    with open(cassette_path, 'a') as cassette_file:
+        cassette_file.write(yaml.dump(data, Dumper=Dumper))