More compact JSON dumps for unicode strings (#38)

Pass `ensure_ascii=False` to `json.dumps`, this will produce shorter json dumps.

itsdangerous.py

@@ -46,7 +46,7 @@ def loads(self, payload):
         return json.loads(payload)
 
     def dumps(self, obj):
-        return json.dumps(obj, separators=(',', ':'))
+        return json.dumps(obj, ensure_ascii=False, separators=(',', ':'))
 
 
 compact_json = _CompactJSON()