Rename chat membership fields of the Message objects

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.
