Add the methods from ActivatorUtilities as ITypeActivator extension methods

Prereq for https://github.com/aspnet/DependencyInjection/issues/33

@halter73 @davidfowl 
This might actually be a carryover from when we threw for missing service types. I don't think we need the try/catch anymore.

I also think we should null check the IServiceProvider since we are doing the same for the ITypeActivator. It would be one thing if we were just passing it through to another method, but we're not.