Add icons and startup images for iOS devices.

BrowserQuest works great as a mobile web app, but it doesn't specify icons (`apple-touch-icon`) or startup images (`apple-touch-startup-image`). I made some rudimentary icons and startup images to fill the gap. I also linked to them from the client's `index.html`.

I've tested my changes in the iOS simulator with iOS 5.0 on the iPad, iPhone, and iPhone (Retina). I also tested it with a physical iPad (Retina).