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.

tests/integration/test_register_serializer.py

@@ -10,7 +10,7 @@ class MockSerializer(object):
     def deserialize(self, cassette_string):
         self.serialize_count += 1
         self.cassette_string = cassette_string
-        return ([], [])
+        return []
 
     def serialize(self, cassette_dict):
         self.deserialize_count += 1