Access to IMvxValueConverter from native wrapper

Hi,

I added a protected read-only property to `MvxNativeValueConverter` to provide access to the wrapped protable `IMvxValueConverter`.

With this change it is possible to create portable converters with public properties which can be used configure the behavior of converters. A simple example:

    public class BoolToTextConverter : MvxValueConverter<bool, string>
    {
        public string TrueValue { get; set; }
        public string FalseValue { get; set; }

        protected override string Convert(bool value, Type targetType, object parameter, CultureInfo culture)
        {
            return value ? TrueValue : FalseValue;
        }
    }

The native wrapper can now look like this:

    public class NativeBoolToTextConverter :
        MvxNativeValueConverter<BoolToTextConverter>
    {
        public string TrueValue
        {
            get { return Wrapped.TrueValue; }
            set { Wrapped.TrueValue = value; }
        }

        public string FalseValue
        {
            get { return Wrapped.FalseValue; }
            set { Wrapped.FalseValue = value; }
        }
    }

This is how it would be instantiated in XAML:

    <conv:NativeBoolToTextConverter x:Key="OnOffConverter" TrueValue="On" FalseValue="Off" />

For other binding modes one can always create derived converters which initialize the properties in their constructor as required:

    public OnOffConverter : BoolToTextConverter
    {
        public OnOffConverter
        {
            TrueValue = "On";
            FalseValue = "Off";
        }
    }