Add ability for OrchardHost to reset its state when Extensions change

OrchardHost calls into all extension loaders to all extensions descriptor,
allowing each loader to add the set of resource to monitor for change.
OrchardHost resets its list of shells at the beginning of a request if
any monitored resources has changed.

--HG--
branch : dev

src/Orchard.Tests/Environment/Extensions/ExtensionManagerTests.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Linq;
 using Autofac;
 using NUnit.Framework;
+using Orchard.Caching;
 using Orchard.Environment.Extensions;
 using Orchard.Environment.Extensions.Folders;
 using Orchard.Environment.Extensions.Loaders;
@@ -57,6 +58,10 @@ namespace Orchard.Tests.Environment.Extensions {
                 return new ExtensionEntry { Descriptor = entry.Descriptor, ExportedTypes = new[] { typeof(Alpha), typeof(Beta), typeof(Phi) } };
             }
 
+            public void Monitor(ExtensionDescriptor descriptor, Action<IVolatileToken> monitor) {
+                throw new NotImplementedException();
+            }
+
             #endregion
         }