Fixes PhysicalFileSystem to use IOHelper instead of HostingEnvironment. Removes all references to new IFileSystem's that canont be supported currently, leaving only the media file system.

Updates all code to use PhysicalFileSystem in place of what 'could' eventually be an abstracted IFileSystem. Updates unit tests
accordingly.

src/Umbraco.Core/IO/PhysicalFileSystem.cs

@@ -18,7 +18,7 @@ namespace Umbraco.Core.IO
 
         public PhysicalFileSystem(string virtualRoot)
         {
-            RootPath = System.Web.Hosting.HostingEnvironment.MapPath(virtualRoot);
+            RootPath = IOHelper.MapPath(virtualRoot);
             _rootUrl = VirtualPathUtility.ToAbsolute(virtualRoot);
         }