The idea was nice, but it really complicated things for us and for the
user.
If a user wants to run more than one handler on an update, he can put
the handlers in different groups or he can have a single handler.
If a user wants to have multiple handlers in the same group which only
one of them should run on the update, he should use check_update().
Since we haven't released this code yet, there's no problem with
backward compatability.
Required fixes:
- CallbackQuery is now comparable.
- Message.effective_attachment, Message.photo,
Message.new_chat_members, Message.new_chat_photo &
Game.text_entitties semantic fixes - when they are not defined,
return an empty list.
- Docstring fix to Update class.
