clear previous callbacks on execute

avoids mixed up display of multiple executions in one output area
when a cell is resubmitted.

closes ipython#4223

IPython/html/static/notebook/js/codecell.js

@@ -66,6 +66,7 @@ var IPython = (function (IPython) {
         this.input_prompt_number = null;
         this.collapsed = false;
         this.cell_type = "code";
+        this.last_msg_id = null;
 
 
         var cm_overwrite_options  = {
@@ -243,14 +244,17 @@ var IPython = (function (IPython) {
         this.output_area.clear_output(true, true, true);
         this.set_input_prompt('*');
         this.element.addClass("running");
+        if (this.last_msg_id) {
+            this.kernel.clear_callbacks_for_msg(this.last_msg_id);
+        }
         var callbacks = {
             'execute_reply': $.proxy(this._handle_execute_reply, this),
             'output': $.proxy(this.output_area.handle_output, this.output_area),
             'clear_output': $.proxy(this.output_area.handle_clear_output, this.output_area),
             'set_next_input': $.proxy(this._handle_set_next_input, this),
             'input_request': $.proxy(this._handle_input_request, this)
         };
-        var msg_id = this.kernel.execute(this.get_text(), callbacks, {silent: false, store_history: true});
+        this.last_msg_id = this.kernel.execute(this.get_text(), callbacks, {silent: false, store_history: true});
     };
 
     /**