Add warning to downloads page

People keep expecting Glowstone to work for production environments. This is not true (yet!)

Images of the error (cause I'm too damn lazy to expose the server port):

- [Chrome](http://i.turt2live.com/w94N.png)
- [Firefox](http://i.turt2live.com/wBRX.png)
- [IE11](http://i.turt2live.com/0i5P.png)

And for those too lazy to click a link:
![image](https://cloud.githubusercontent.com/assets/1190097/4604224/08f0d6e8-518d-11e4-981a-b122d6e7fffc.png)

Edit: Here's a live preview: http://home.turt2live.com:4000