rschat: Manage multiple Web based chat rooms

This class can be used to manage multiple chat rooms.

The records of the users, chat rooms and the messages sent by the users are stored in a MySQL database.

This class can:

- List the existing chat rooms:
- Check if a given chat room was already created
- Send a message to a given room on behalf of a given user
- Get all the messages sent to a room since a given message number