Fixes the custom field sorting, no longer has a sub query for order by, now supports both mysql and sqlce, adds a unit test (should add more though)

src/Umbraco.Core/Persistence/SqlSyntax/ISqlSyntaxProvider.cs

@@ -69,8 +69,7 @@ namespace Umbraco.Core.Persistence.SqlSyntax
         bool SupportsClustered();
         bool SupportsIdentityInsert();
         bool? SupportsCaseInsensitiveQueries(Database db);
-
-        string IsNull { get; }
+        
         string ConvertIntegerToOrderableString { get; }
         string ConvertDateToOrderableString { get; }
         string ConvertDecimalToOrderableString { get; }