Import statements, message function

[When adding a new command to circus](https://github.com/sebschlicht/bachelor-thesis/wiki/HowTo:-Create-a-custom-circus-command) I ran into several issues:

1. missing import statements (Command, ArgumentError, MessageError)
2. NotImplementedError: message function not implemented

I browsed your code looking for an example and to get an idea of `message`'s purpose.
This pull requests adds the missing import statements and the `message` function together with a brief explanation to the code example.

Please check if I got the function's purpose.