Tempo is not broadcasted to clients

[DerouineauNicolas]

To be fixed.

[xavierfav]

In server.js:

• Add tempo propriety in sequncerState.
• Add socket.on('tempo', function (message) {...}) method for receiving tempo changes, changing the tempo in the according sequencer state from sequencerStates and broadcasting the tempo. This function should be declared in the callback of socket.on('room', ... ) corresponding to a room connection.

In index.ejs:

• Add sendTempo(trackId, tempo) function that emit a 'tempo' socket to the server.
• Add socket.on('sendTempo', function (message) {...}) that changes the tempo in the client sequencer.

In app.js:

• Call sendTempo(trackId, tempo) in changeTempoListener().