Exposes session store, cookie parser, and session key.

Changes:
1. Exposes session store, cookie parser, and session key which allows session.socket.io to match the current session with the socket.io connection in that session.
2. Add a switch to enable the turning off of the view engine cache in development mode. This helps when debugging other engines like swig.

Keystone session.socket.io example:

```javascript
keystone.start(
{
  onHttpServerCreated: function()
  {
    keystone.app.io = require('socket.io').listen(keystone.httpServer);
    keystone.app.sessionsocketio = new require('session.socket.io')(keystone.app.io, keystone.app.sessionOpts.store, keystone.app.sessionOpts.cookieParser, keystone.app.sessionOpts.key);
    keystone.app.sessionsocketio.on('connection', function (err, socket, session) {
      socket.emit('alert', { message: 'socket.io is connected to session: ' + session });
    });
  }
});
```

Socket.io v1.x HTML example:

```
<script src="/socket.io/socket.io.js"></script>
<script>
  var socket = io('http://localhost');
  socket.on('alert', function (data) {
    console.log(data.message);
  });
</script>
```