set "mode-class" property of twittering mode to "special"

from http://www.gnu.org/software/emacs/manual/html_node/elisp/Major-Mode-Conventions.html :

> If mode is appropriate only for specially-prepared text produced by the mode itself (rather than by the user typing at the keyboard or by an external file), then the major mode command symbol should have a property named mode-class with value special.