change serializer API so it doesn't have to do file stuff

vcr/serializers/yamlserializer.py

@@ -7,16 +7,16 @@ except ImportError:
     from yaml import Loader, Dumper
 
 
-def load(cassette_path):
-    data = yaml.load(open(cassette_path), Loader=Loader)
+def deserialize(cassette_string):
+    data = yaml.load(cassette_string, Loader=Loader)
     requests = [r['request'] for r in data]
     responses = [r['response'] for r in data]
     return requests, responses
 
 
-def dumps(requests, responses):
+def serialize(cassette_dict):
     data = ([{
         'request': request,
         'response': response,
-    } for request, response in zip(requests, responses)])
+    } for request, response in zip(cassette_dict['requests'], cassette_dict['responses'])])
     return yaml.dump(data, Dumper=Dumper)