Work in progress portable library.

ReactiveCommand and ReactiveAsyncCommand moved to RxUI-Core
Renamed ReactiveUI_Portable to ReactiveUI.

Added _Net40 to .Net 4.0 projects.
Moved Validation.cs into RxUI-Xaml.
Updated projects to reference the Portable Library.
Lazy in its own file.
Linked Command classes and Interfaces into RxUI-Xaml for .Net 4.0
Fixed compile errors.

Fixed InternalsVisibleTo.
Temporarily handle unit test detection by returning false.
Change the namespace for PropertyChanging classes to work around namespace collision on .Net45. Need to think about the best way to handle this.