support for .NET framework 3.5

Still have bugs

Net35/HandyControlDemo_Net35/ViewModel/ImageBrowserDemoViewModel.cs

@@ -0,0 +1,15 @@
+using System;
+using GalaSoft.MvvmLight.Command;
+using HandyControl.Controls;
+
+namespace HandyControlDemo.ViewModel
+{
+    public class ImageBrowserDemoViewModel
+    {
+        private RelayCommand _openImgCmd;
+
+        public RelayCommand OpenImgCmd =>
+            _openImgCmd ?? (_openImgCmd = new RelayCommand(() =>
+                new ImageBrowser(new Uri("pack://application:,,,/Resources/Img/1.jpg")).Show()));
+    }
+}