Make Serializers Dumber

Let's have the serializer just worry about serializing the dict
that we hand it, and move the unicode stuff up to a serialize module.

This should hopefully let us move toward using a version string in
cassettes.

vcr/serialize.py

@@ -0,0 +1,33 @@
+from serializers import compat
+from vcr.request import Request
+
+"""
+Just a general note on the serialization philosophy here:
+I prefer cassettes to be human-readable if possible.  Yaml serializes
+bytestrings to !!binary, which isn't readable, so I would like to serialize to
+strings and from strings, which yaml will encode as utf-8 automatically.
+All the internal HTTP stuff expects bytestrings, so this whole serialization
+process feels backwards.
+
+Serializing: bytestring -> string (yaml persists to utf-8)
+Deserializing: string (yaml converts from utf-8) -> bytestring
+"""
+
+
+def deserialize(cassette_string, serializer):
+    data = serializer.deserialize(cassette_string)
+    requests = [Request._from_dict(r['request']) for r in data]
+    responses = [r['response'] for r in data]
+    responses = [compat.convert_to_bytes(r['response']) for r in data]
+    return requests, responses
+
+
+def serialize(cassette_dict, serializer):
+    data = ([{
+        'request': request._to_dict(),
+        'response': compat.convert_to_unicode(response),
+    } for request, response in zip(
+        cassette_dict['requests'],
+        cassette_dict['responses'],
+    )])
+    return serializer.serialize(data)