Add next_token field in tasks query

Author: Yuan Yao

scaleapi/__init__.py

@@ -119,7 +119,8 @@ def tasks(self, **kwargs):
         offset is the number of results to skip (for showing more pages).
         """
         allowed_kwargs = {'start_time', 'end_time', 'status', 'type', 'project',
-                          'batch', 'limit', 'offset', 'completed_before', 'completed_after'}
+                          'batch', 'limit', 'offset', 'completed_before', 'completed_after', 
+                          'next_token'}
         for key in kwargs:
             if key not in allowed_kwargs:
                 raise ScaleInvalidRequest('Illegal parameter %s for ScaleClient.tasks()'