add support for "new" keyword and override properties

[odyth]

instead of just throwing an exception in VerifyKeyUniqueness if two properties have the same name, lets first see if those objects inherit from one another. This will let you have a virtual property that is override and use the new keyword. We take the derived most class in this case and use that for serialization.

[yfakariya]

Thank you for nice PR, but I've failed to merge because it looks that it does not solve hiding problem...

1. Could you push or post test cases you used?
2. Can I interpret this PR that it solves the problem the serializer ignore re-defined (hidden or marked with new) members in derived class? If so, VerifyKeyUniqueness is not suitable because it only handle members marked with custom attributes.

I tried to test your PR with following types:

    public class HidingBase
    {
        public string SameType { get; set; }
        public string ShouldBeInt32 { get; set; }
    }

    public class HidingBaseMarked
    {
        [MessagePackMember( 0 )]
        public string SameType { get; set; }

        [MessagePackMember( 1 )]
        public string ShouldBeInt32 { get; set; }
    }

    public class HidingNotMarked : HidingBase
    {
        public new string SameType { get; set; }
        public new int ShouldBeInt32 { get; set; }
    }

    public class HidingMarkedOnDerived : HidingBase
    {
        [MessagePackMember( 0 )]
        public new string SameType { get; set; }

        [MessagePackMember( 1 )]
        public new int ShouldBeInt32 { get; set; }
    }

    public class HidingMarkedOnBase : HidingBaseMarked
    {
        public new string SameType { get; set; }

        public new int ShouldBeInt32 { get; set; }
    }

    public class HidingMarkedOnBoth : HidingBaseMarked
    {
        [MessagePackMember( 0 )]
        public new string SameType { get; set; }

        [MessagePackMember( 1 )]
        public new int ShouldBeInt32 { get; set; }
    }

[yfakariya]

This PR looks solve same as #161, so close this PR.
Thank you for great work and sorry for too-late response/discussion.