adding IMvxNotifyPropertyChanged extension methods 

to simplify notification property setters

e.g., 
```csharp
        private string _hello = default(string);
        public string Hello
        {
            get { return _hello; }
            set { this.RaiseAndSetIfChanged(ref _hello, value, () => Hello); }
        }
```

Based on the work of paulcbetts in the reactiveui project
See https://github.com/reactiveui/ReactiveUI/blob/rxui6-master/ReactiveUI/ReactiveObject.cs