Rxui5 validation

Took a stab at implemeting a validation framework for ReactiveUI as per discussed in https://github.com/reactiveui/ReactiveUI/pull/217

In essence, it implements two mechanisms:
- On one hand, we have `IBindingErrorProvider`, that provides a stream of errors for a given object. An implementation exist for both `IDataErrorInfo` and `INotifyDataErrorInfo`, and more can be added (similarly to the extensibility of the binding mechanism).
- On the other hand, we have the `IBindingDisplayProvider`, that displays the current error on the view. An implementation is provided for WPF that basically hacks `System.ComponentModel.Validation` to display the error. In the future, we should look to move towards a more general system for all the xaml frameworks. This is also extensible and pluggable.

The validation framework then takes care of wiring these two components up, so that everything is handled correctly (I hope). The syntax looks as follows:
```c#
public class MyView : Window, IViewFor<ViewModel>
{
  public MyView()
  {
    InitializeComponent();
    
    this.Bind(ViewModel,  x => x.SomeProperty, v => v.MyTextBox.Text);
    this.DisplayValidationFor(x => x.MyTextBox.Text) // enables validation
  }
}
```
and a sample can be found at https://github.com/wendazhou/ReactiveUI/tree/rxui5-validation-sample

It compiles on most platforms (could not test WP8), but will not work on any except WPF as there is no implementation of `IBindingDisplayProvider` on both.

Also, the code makes it very clear moving to portable libraries #217 will be very beneficial, and figuring out a way to sort out the `RxApp` and introducing some sort of dependency injection would be really great (as per https://github.com/reactiveui/ReactiveUI/pull/219).

There is one known issue for the moment: in order to be able to find all the bindings on a given view, I register a `IPropertyBindingHook` that keeps track of *all* the bindings *ever created*... Not only does this probably leak, it is not very efficient, and hard to configure (we need it on by default so that it registers all bindings, but then if the validation is not used, it consumes memory purposelessly).

So, in conclusion, I'd like to gather some feedback on the design, and adress the following:
- Provide implementations of `IBindingDisplayProvider` on all platforms
- Figure out how to disable the registration hook if user is not using the framework
- Test the whole thing, but will be easier with better DI
- reduce the huge mess of `#if` by using portable libraries and better DI

@jlaanstra I saw that you rebuilt `ReactiveValidatedObject` for for `INotifyErrorInfo`, do you have any input on that?

In any case, thanks a lot @xpaulbettsx for the great work!