Hosting rework #2

- Merge HostingContext and HostingEngine
- Cleanup usage via builder pattern

src/Microsoft.AspNet.Hosting.Interfaces/IHostingEnvironment.cs

@@ -7,10 +7,13 @@ namespace Microsoft.AspNet.Hosting
 {
  public interface IHostingEnvironment
  {
+ // This must be settable!
  string EnvironmentName { get; set; }
 
- string WebRootPath { get; }
+ // This must be settable!
+ string WebRootPath { get; set; }
 
- IFileProvider WebRootFileProvider { get; }
+ // This must be settable!
+ IFileProvider WebRootFileProvider { get; set; }
  }
 }

src/Microsoft.AspNet.Hosting/ApplicationLifetime.cs

@@ -14,10 +14,6 @@ public class ApplicationLifetime : IApplicationLifetime
  private readonly CancellationTokenSource _stoppingSource = new CancellationTokenSource();
  private readonly CancellationTokenSource _stoppedSource = new CancellationTokenSource();
 
- public ApplicationLifetime()
- {
- }
-
  /// <summary>
  /// Triggered when the application host is performing a graceful shutdown.
  /// Request may still be in flight. Shutdown will block until this event completes.