Add ability to generate secret dynamically

This allow you to pass a function instead of an string in order to
generate secret based on the new connection features.

test/fixture/secret_function.js

@@ -0,0 +1,89 @@
+var express = require('express');
+var http = require('http');
+
+var socketIo = require('socket.io');
+var socketio_jwt = require('../../lib');
+
+var jwt = require('jsonwebtoken');
+
+var xtend = require('xtend');
+var bodyParser = require('body-parser');
+
+var server, sio;
+var enableDestroy = require('server-destroy');
+
+exports.start = function (options, callback) {
+  var SECRETS = {
+    123: 'aaafoo super sercret',
+    555: 'other'
+  };
+
+  if(typeof options == 'function'){
+    callback = options;
+    options = {};
+  }
+
+  options = xtend({
+    secret: function(request, decodedToken, callback) {
+      callback(null, SECRETS[decodedToken.id]);
+    },
+    timeout: 1000,
+    handshake: true
+  }, options);
+
+  var app = express();
+
+  app.use(bodyParser.json());
+
+  app.post('/login', function (req, res) {
+    var profile = {
+      first_name: 'John',
+      last_name: 'Doe',
+      email: 'john@doe.com',
+      id: req.body.username === 'valid_signature' ? 123 : 555
+    };
+
+    // We are sending the profile inside the token
+    var token = jwt.sign(profile, SECRETS[123], { expiresIn: 60*60*5 });
+
+    res.json({token: token});
+  });
+
+  server = http.createServer(app);
+
+  sio = socketIo.listen(server);
+
+  if (options.handshake) {
+    sio.use(socketio_jwt.authorize(options));
+
+    sio.sockets.on('echo', function (m) {
+      sio.sockets.emit('echo-response', m);
+    });
+  } else {
+    sio.sockets
+      .on('connection', socketio_jwt.authorize(options))
+      .on('authenticated', function (socket) {
+        socket.on('echo', function (m) {
+          socket.emit('echo-response', m);
+        });
+      });
+  }
+
+  server.__sockets = [];
+  server.on('connection', function (c) {
+    server.__sockets.push(c);
+  });
+
+  server.listen(9000, callback);
+  enableDestroy(server);
+};
+
+exports.stop = function (callback) {
+  sio.close();
+  try {
+    server.destroy();
+  } catch (er) {}
+
+  callback();
+};
+