Basic room system

[sknebel]

First larger step for the backend is to implement basic rooms that allow a group of clients to communicate. (compare meet-here/meet-here#13, meet-here/meet-here#14)

• allow a client to create a new room
• allow a client to join an existing room
• allow a client to send a message that is received by all clients in the room

The system should be able to verify to contents of the message (= forbid that someone claims to be someone else/reject wrongly formatted data)