Further integration with Django builtin generic class-based views

* Backwards-compatible refactoring of AjaxTempateView to provide
 AjaxTemplateMixin for more customized views.

+ AjaxTemplateMixin subclasses are now able to gain more control
  over decision of what kind of reply should be served to particular
  request by overriding self.trigger_ajax_response() method.

+ AjaxTemplateMixin now calls self.get_ajax_template_postfix() to
  retrieve ajax template postfix. You can now serve a lot of similar
  templates through one view, deriving ajaxified templates filenames
  programmatically.