Added context to filter

This change will allow us passing a context for the filter function, for example:

    Controller.fetchData = function () {
      Collections.tasks.partialFetch({
        filter: this.filter
      , context: { selected_project_id: this.selected_items.project_id }
      , success: this.onSuccess
      });
    };
    
    Controller.filter = function (item) {
      return item.get('project_id') === this.selected_project_id;
    };