Add ArgumentsProvider for Windows Phone 8.

This PR adds an implementation of `IArgumentsProvider` for Windows Phone 8.

## Why do we need this?

The startup logic of ReactiveApp needs to have access to any arguments passed to the app to determine which viewmodel should be shown. On Windows Phone Silverlight these arguments are part of the navigation Uri. This class unwraps the arguments in the Uri and publishes them on the observable.

## How does it work?

There are two cases we need to handle.

* A new instance: Cancel the initial navigation and pass the arguments.
* An existing instance: We will receive a reset navigation followed by the navigation with the arguments. We cancel the second navigation and pass the arguments on.