only consider stream output for data rate limit

large stream outputs cause much more problems than image output

this does open up to large HTML and/or displayed text output,
but those seem to behave well more often than they don't.

Author: minrk

notebook/notebookapp.py

@@ -997,7 +997,7 @@ def _update_server_extensions(self, change):
         limited.""")
 
     iopub_data_rate_limit = Float(1000000, config=True, help="""(bytes/sec)
-        Maximum rate at which messages can be sent on iopub before they are
+        Maximum rate at which stream output can be sent on iopub before they are
         limited.""")
 
     rate_limit_window = Float(3, config=True, help="""(sec) Time window used to 

notebook/services/kernels/handlers.py

@@ -336,7 +336,10 @@ def write_stderr(error_message):
 
             # Increment the bytes and message count
             self._iopub_window_msg_count += 1
-            byte_count = sum([len(x) for x in msg_list])
+            if msg_type == 'stream':
+                byte_count = sum([len(x) for x in msg_list])
+            else:
+                byte_count = 0
             self._iopub_window_byte_count += byte_count
 
             # Queue a removal of the byte and message count for a time in the