Resolves #258 Timer support

lib/entity_class/const.dart

@@ -28,6 +28,7 @@ class EntityState {
   static const unavailable = 'unavailable';
   static const ok = 'ok';
   static const problem = 'problem';
+  static const active = 'active';
 }
 
 class EntityUIAction {

lib/entity_class/timer_entity.dart

@@ -0,0 +1,36 @@
+part of '../main.dart';
+
+class TimerEntity extends Entity {
+  TimerEntity(Map rawData) : super(rawData);
+
+  Duration duration;
+
+  @override
+  void update(Map rawData) {
+    super.update(rawData);
+    String durationSource = "${attributes["duration"]}";
+    List<String> durationList = durationSource.split(":");
+    if (durationList.length == 1) {
+      duration = Duration(seconds: int.tryParse(durationList[0] ?? 0));
+    } else if (durationList.length == 2) {
+      duration = Duration(
+          hours: int.tryParse(durationList[0]) ?? 0,
+          minutes: int.tryParse(durationList[1]) ?? 0
+      );
+    } else if (durationList.length == 3) {
+      duration = Duration(
+          hours: int.tryParse(durationList[0]) ?? 0,
+          minutes: int.tryParse(durationList[1]) ?? 0,
+        seconds: int.tryParse(durationList[2]) ?? 0
+      );
+    } else {
+      Logger.e("Cann't parse $entityId duration: $durationSource");
+      duration = Duration(seconds: 0);
+    }
+  }
+
+  @override
+  Widget _buildStatePart(BuildContext context) {
+    return TimerState();
+  }
+}