Enable use of value types as for models.

The model class restriction prevents the creation of editor and display templates for value types, which prevents adding a date picker as show in this example: http://www.asp.net/mvc/tutorials/javascript/using-the-html5-and-jquery-ui-datepicker-popup-calendar-with-aspnet-mvc/using-the-html5-and-jquery-ui-datepicker-popup-calendar-with-aspnet-mvc-part-4