Tweak dynamic extension loader

* Add "Assembly Src" directive to WebForms files loaded from vpp
* Notify the OrchardHost when the .csproj file a a dynamic module is changed.
* Make the registration of custom virtual path provider a bit more extensible,
  so that the custom VPP can be instantiated using DI.

--HG--
branch : dev

src/Orchard.Web/Global.asax.cs

@@ -31,7 +31,6 @@ namespace Orchard.Web {
 
             RegisterRoutes(RouteTable.Routes);
 
-            HostingEnvironment.RegisterVirtualPathProvider(new WebFormsExtensionsVirtualPathProvider());
             _host = OrchardStarter.CreateHost(MvcSingletons);
             _host.Initialize();