adding IMvxNotifyPropertyChanged extension methods to simplify notification property setters

e.g., 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
Merge branch 'RaiseAndSetIfChanged-Extensions' of github.com:patsissons/MvvmCross into 3.5