add Cell.is_mergeable method

and check it in the Notebook merge methods

IPython/html/static/notebook/js/cell.js

@@ -207,7 +207,7 b' var IPython = (function (IPython) {'
 
 
     /**
-     * can the cell be splitted in 2 cells.
+     * can the cell be split into two cells
      * @method is_splittable
      **/
     Cell.prototype.is_splittable = function () {
@@ -216,6 +216,15 b' var IPython = (function (IPython) {'
 
 
     /**
+     * can the cell be merged with other cells
+     * @method is_mergeable
+     **/
+    Cell.prototype.is_mergeable = function () {
+        return true;
+    };
+
+
+    /**
      * @return {String} - the text before the cursor
      * @method get_pre_cursor
      **/

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

@@ -1175,8 +1175,14 b' var IPython = (function (IPython) {'
     Notebook.prototype.merge_cell_above = function () {
         var index = this.get_selected_index();
         var cell = this.get_cell(index);
+        if (!cell.is_mergeable()) {
+            return;
+        }
         if (index > 0) {
             var upper_cell = this.get_cell(index-1);
+            if (!upper_cell.is_mergeable()) {
+                return;
+            }
             var upper_text = upper_cell.get_text();
             var text = cell.get_text();
             if (cell instanceof IPython.CodeCell) {
@@ -1199,8 +1205,14 b' var IPython = (function (IPython) {'
     Notebook.prototype.merge_cell_below = function () {
         var index = this.get_selected_index();
         var cell = this.get_cell(index);
+        if (!cell.is_mergeable()) {
+            return;
+        }
         if (index < this.ncells()-1) {
             var lower_cell = this.get_cell(index+1);
+            if (!lower_cell.is_mergeable()) {
+                return;
+            }
             var lower_text = lower_cell.get_text();
             var text = cell.get_text();
             if (cell instanceof IPython.CodeCell) {