make config options actually do stuff

README.md

@@ -116,12 +116,12 @@ import vcr
 
 MySerializer(object):
     """
-    Must implement serialize() and deserialize() functions
+    Must implement serialize() and deserialize() classmethods
     """
     pass
 
 my_vcr = VCR()
-my_vcr.register_serializer(MySerializer)
+my_vcr.register_serializer('my', MySerializer)
 ```
 
 ##Installation

vcr/cassette.py

@@ -21,7 +21,7 @@ class Cassette(object):
         new_cassette._load()
         return new_cassette
 
-    def __init__(self, path, serializer="yaml"):
+    def __init__(self, path, serializer):
         self._path = path
         self._serializer = serializer
         self.data = OrderedDict()

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
+

vcr/persist.py

@@ -1,21 +1,8 @@
 from .persisters.filesystem import FilesystemPersister
-from .serializers.yamlserializer import YamlSerializer
-from .serializers.jsonserializer import JSONSerializer
-
-def _get_serializer_cls(serializer):
-    serializer_cls = {
-        'yaml': YamlSerializer,
-        'json': JSONSerializer,
-    }.get(serializer)
-    if not serializer_cls:
-        raise ImportError('Invalid serializer %s' % serializer)
-    return serializer_cls
 
 def load_cassette(cassette_path, serializer):
-    serializer_cls = _get_serializer_cls(serializer)
-    return serializer_cls.load(cassette_path)
-    
+    return serializer.load(cassette_path)
+
 def save_cassette(cassette_path, requests, responses, serializer):
-    serializer_cls = _get_serializer_cls(serializer)
-    data = serializer_cls.dumps(requests, responses)
+    data = serializer.dumps(requests, responses)
     FilesystemPersister.write(cassette_path, data)