Add the methods from ActivatorUtilities as ITypeActivator extension methods

Prereq for https://github.com/aspnet/DependencyInjection/issues/33

@halter73 @davidfowl 
```
Hosting\src\Microsoft.AspNet.Hosting\Startup\StartupLoader.cs
Hosting\src\Microsoft.AspNet.Hosting\Server\ServerManager.cs
EntityFramework\src\EntityFramework.Commands\ContextTool.cs
```