add 'running' class to running code cells

Will allow special styling of code cells that are running via
`div.code_cell.running` in css.  Currently, no change is made.

IPython/frontend/html/notebook/static/css/notebook.css

@@ -191,6 +191,9 b' div.cell {'
 div.code_cell {
     background-color: white;
 }
+/* any special styling for code cells that are currently running goes here */
+div.code_cell.running {
+}
 
 div.prompt {
     /* This needs to be wide enough for 3 digit prompt numbers: In[100]: */

IPython/frontend/html/notebook/static/js/notebook.js

@@ -691,6 +691,7 b' var IPython = (function (IPython) {'
         var cell = this.cell_for_msg(reply.parent_header.msg_id);
         if (msg_type === "execute_reply") {
             cell.set_input_prompt(content.execution_count);
+            cell.element.removeClass("running");
             this.dirty = true;
         } else if (msg_type === "complete_reply") {
             cell.finish_completing(content.matched_text, content.matches);
@@ -830,6 +831,7 b' var IPython = (function (IPython) {'
         if (cell instanceof IPython.CodeCell) {
             cell.clear_output(true, true, true);
             cell.set_input_prompt('*');
+            cell.element.addClass("running");
             var code = cell.get_code();
             var msg_id = that.kernel.execute(cell.get_code());
             that.msg_cell_map[msg_id] = cell.cell_id;