Rename chat membership fields of the Message objects #60

The recent update of the Bots API renamed some of the filelds in the Message object. While the old ones will probably keep working for some time, renaming them is a thing I want to do because the old name is plain ugly.

• new_chat_participant > new_chat_member
• left_chat_participant > left_chat_member

I'll ensure backward compatibility for this change until botogram 1.0.