studio html editor - styled <pre> and <code> elements, revised code tinyMCE icon

cms/static/css/tiny-mce.css

@@ -117,6 +117,7 @@ code {
     font-family: monospace, serif;
     background: none;
     color: #3c3c3c;
+    padding: 0;
 }
 
 table {

common/lib/xmodule/xmodule/css/html/display.scss

@@ -100,6 +100,7 @@ code {
   color: $baseFontColor;
   font-family: monospace, serif;
   background: none;
+  padding: 0;
 }
 
 table {

common/lib/xmodule/xmodule/js/src/html/edit.coffee

@@ -51,7 +51,7 @@ class @HTMLEditingDescriptor
   @setupTinyMCE: (ed) ->
     ed.addButton('wrapAsCode', {
       title : 'Code Block',
-      image : '/static/images/code.gif',
+      image : '/static/images/ico-tinymce-code.png',
       onclick : () ->
         ed.formatter.toggle('code')
     })