title: MvvmCross: NotImplementedException calling EnsureFolderExists method of IMvxFileStore
body: <p>I'm developing my first Windows Store App, using MvvmCross framework and I have a problem with images management. In particular I have the following simple ViewModel in my PCL project, and a Store project with a button bound with AddPictureCommand.</p>
  
  <pre><code>    public class FirstViewModel : MvxViewModel
  {
  
      IMvxPictureChooserTask _pictureChooserTask;
      IMvxFileStore _fileStore;
  
      public FirstViewModel(IMvxPictureChooserTask pictureChooserTask, IMvxFileStore fileStore)
      {
          _pictureChooserTask = pictureChooserTask;
          _fileStore = fileStore;
      }
  
      private byte[] _pictureBytes;
      public byte[] PictureBytes
      {
          get { return _pictureBytes; }
          set
          {
              if (_pictureBytes == value) return;
              _pictureBytes = value;
              RaisePropertyChanged(() =&gt; PictureBytes);
          }
      }
  
      public ICommand AddPictureCommand
      {
          get { return new MvxCommand(() =&gt; 
          {
              _pictureChooserTask.ChoosePictureFromLibrary(400, 95, pictureAvailable, () =&gt; { });
          }); }
      }
  
      private void pictureAvailable(Stream stream)
      {
          MemoryStream memoryStream = new MemoryStream();
          stream.CopyTo(memoryStream);
          PictureBytes = memoryStream.ToArray();
  
          GenerateImagePath();
      }
  
      private string GenerateImagePath()
      {
          if (PictureBytes == null) return null;
          var RandomFileName = "Image" + Guid.NewGuid().ToString("N") + ".jpg";
          _fileStore.EnsureFolderExists("Images");
          var path = _fileStore.PathCombine("Images", RandomFileName);
          _fileStore.WriteFile(path, PictureBytes);
  
          return path;
      }
  }
  </code></pre>
  
  <p>The problem is that the method <strong>_fileStore.EnsureFolderExists("Images");</strong>
  gives me the an "NotImplementedException" with message: "Need to implement this - doesn't seem obvious from the StorageFolder API".
  Has anyone already seen it before?
  Thank you</p>