I REACHED 99% YAY.

 Emit allPlayersReady after updateSettings on playerLeft if all players still present are ready

See merge request !5

BanchoLobby declaration should extend EventEmitter

The implementation of BanchoLobby extends EventEmitter, but the type declaration was missing this.
With this change, EventEmitter properties can be accessed, such as `off` for removing events.

Doesn't seem very reliable, and is not applied in every case.

It ignores gamemode and an accurate check is impossible to implement.

As reliable as Bancho allows it to be, unfortunately.

Handle cases where getPlayerSlot() returns undefined

When bancho.js joins an existing lobby, you can receive playerMoved or
playerLeft events *before* bancho.js had the time to fetch the lobby
info. When that happens, an unhandled error is thrown, since "undefined"
is not a valid key for the slots array.

I chose to just check if getPlayerSlot() returns something, but a better
fix might be to ignore those events until the lobby is initialized.

Check if member exists before emitting it

More or less IRC compliant, like all IRC things.
See https://tools.ietf.org/id/draft-oakley-irc-ctcp-01.html#rfc.appendix.A.1