Making errors argument optionnal

issue #121

schema.py

@@ -16,7 +16,8 @@
 class SchemaError(Exception):
     """Error during Schema validation."""
 
-    def __init__(self, autos, errors):
+    def __init__(self, autos, errors=None):
+        errors = [] if errors is None else errors
         self.autos = autos if type(autos) is list else [autos]
         self.errors = errors if type(errors) is list else [errors]
         Exception.__init__(self, self.code)