Paging infrastructure implemented through specifications. (#151)

* Paging criterias added in BaseSpecification. New paginated specification added (CatalogFilterPaginatedSpecification) . EFRepository cleaned up and paging implementation added. Usage of the new paging infrastructure in CatalogService (Web and WebRazor).

* Use IReadOnlyList<T> instead of List<T> as return type from repositories.

* - Ordering possibility added in the specification.
- Evaluation of the specification placed in separate class.

src/ApplicationCore/Interfaces/ISpecification.cs

@@ -9,5 +9,11 @@ namespace Microsoft.eShopWeb.ApplicationCore.Interfaces
         Expression<Func<T, bool>> Criteria { get; }
         List<Expression<Func<T, object>>> Includes { get; }
         List<string> IncludeStrings { get; }
+        Expression<Func<T, object>> OrderBy { get; }
+        Expression<Func<T, object>> OrderByDescending { get; }
+
+        int Take { get; }
+        int Skip { get; }
+        bool isPagingEnabled { get;}
     }
 }