refactor(typings): export Socket class

In order to be able to cast it on the argument of the "connect" event:

```js
import { Socket } from "socket.io";

io.on("connect", (socket: Socket) => {
  // ...
});
```

Author: darrachequesne

lib/index.ts

@@ -709,3 +709,5 @@ emitterMethods.forEach(function(fn) {
 
 module.exports = (srv?, opts?) => new Server(srv, opts);
 module.exports.Server = Server;
+
+export { Socket }; // for "connect" event typing

test/socket.io.ts

@@ -1,6 +1,6 @@
 "use strict";
 
-import { Server } from "..";
+import { Server, Socket } from "..";
 import { createServer } from "http";
 import fs = require("fs");
 import { join } from "path";
@@ -357,7 +357,7 @@ describe("socket.io", () => {
       const sio = new Server(srv);
       srv.listen(() => {
         const socket = client(srv);
-        sio.on("connection", socket => {
+        sio.on("connection", (socket: Socket) => {
           expect(socket).to.be.a(Socket);
           done();
         });