make config options actually do stuff

vcr/config.py

@@ -1,9 +1,37 @@
 from .cassette import Cassette
+from .serializers.yamlserializer import YamlSerializer
+from .serializers.jsonserializer import JSONSerializer
 
 class VCR(object):
     def __init__(self, serializer='yaml', cassette_library_dir=None):
         self.serializer = serializer
         self.cassette_library_dir = cassette_library_dir
+        self.serializers = {
+            'yaml': YamlSerializer,
+            'json': JSONSerializer,
+        }
+
+    def _get_serializer(self, serializer_name):
+        try:
+            serializer = self.serializers[serializer_name]
+        except KeyError:
+            print "Serializer {0} doesn't exist or isn't registered".format(serializer_name)
+            raise KeyError
+        return serializer
 
     def use_cassette(self, path, **kwargs):
-        return Cassette.load(path, **kwargs)
+        serializer_name = kwargs.get('serializer', self.serializer)
+        cassette_library_dir = kwargs.get('cassette_library_dir', self.cassette_library_dir)
+
+        if cassette_library_dir:
+            path = os.path.join(cassette_library_dir, path)
+
+        merged_config = {
+            "serializer": self._get_serializer(serializer_name),
+        }
+
+        return Cassette.load(path, **merged_config)
+
+    def register_serializer(self, name, serializer):
+        self.serializers[name] = serializer
+