Multispiel - md_sandbox

This is the complete Multispiel implementation.

When used with a traditional game.js and game.html, it should behave exactly the same as usual.
In order to spawn multiple instances, however, the separate game.js files need to be in closures so that they do not affect each other's PS objects, and they need divs set up in the location where they should be created in the page. There are examples of this in the /multispiel/ folder.

Known issues:
* Keyboard focus across multiple instances is not always good
* Haven't tested audio for multiple instances

I expect to have to make some changes to the code, so I will be working on the above issues in the meantime. Any recommendations are welcome.