enables default backspace and delete key when ICE is not tracking

without this if you turn off track changes then the editor does not process deletes and backspaces

lib/tinymce/jscripts/tinymce4/plugins/ice/plugin.js

@@ -37,7 +37,7 @@
 			// make sure only the plugin is handling this event	and not the editor		
 			ed.on('keydown', function(e) {
 				// prevent the delete key and backspace keys from firing twice
-				if(e.keyCode == 46 || e.keyCode==8) {
+				if((e.keyCode == 46 || e.keyCode == 8) && self.isTracking) {
 					e.preventDefault();
 			    }
 			});