Addresses https://github.com/Pita/etherpad-lite/issues/173 but needs a lot of testing and a word of warning, at current doing shift tab deletes the previous character as it emulates a backspace.

static/js/ace2_inner.js

@@ -3579,12 +3579,18 @@ function OUTER(gscope)
   editorInfo.ace_doIndentOutdent = doIndentOutdent;
 
   function doTabKey(shiftDown)
+  {
+    if (shiftDown === true){
+       doDeleteKey();
+    }
+    else
     {
       if (!doIndentOutdent(shiftDown))
       {
         performDocumentReplaceSelection(THE_TAB);
       }
     }
+  }
 
   function doDeleteKey(optEvt)
   {