implement quit api. indicate if backend is offline

static/js/oml.js

@@ -128,6 +128,28 @@
                 } catch(e) {}
             }
             removeScreen();
+            oml.bindEvent({
+                close: function() {
+                    if(!$('#offline').length) {
+                        $('<div>')
+                            .attr({id: 'offline'})
+                            .css({
+                                left: 0,
+                                right: 0,
+                                top: 0,
+                                bottom: 0,
+                                position: 'absolute',
+                                opacity: 0.5,
+                                backgroundColor: 'black',
+                                zIndex: 1000
+                            })
+                            .appendTo(Ox.$body);
+                    }
+                },
+                open: function() {
+                    $('#offline').remove();
+                }
+            });
         });
     }