Virtual Convert and ConvertBack methods in MvxNativeValueConverter

Hi,

I made `Convert` and `ConvertBack` methods in `MvxNativeValueConverter` virtual to make it possible to override them in the native converter class deriving from `MvxNativeValueConverter`.

This is useful when when the native converter needs to return a native class not available in a portable class library, such as `BitmapImage`. Now it's possible to create an `IMvxValueConverter` returning a `Uri` and just pass it to a `BitmapImage` in the native wrapper:

    public override object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var uri =  base.Convert(value, targetType, parameter, culture) as Uri;
        return uri == null ? null : new BitmapImage(uri);
    }

I changed all three implementations of `MvxNativeValueConverter` this time.