Hosting rework #2

- Merge HostingContext and HostingEngine
- Cleanup usage via builder pattern

Author: HaoK

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