Async level loading + few tweaks

[Qaomen]

Changes

• MarbleLevel now loads default level data in the constructor (instead of being manually set before).
• Added a function that uses a worker thread to fetch/parse level data, and loads the level on the main thread afterwards.
• Level data loading has a bit more error checking now.
• game.js will keep track of what level is currently loaded, so it won't unnecessarily reload on reconnect.

On my machine, the async loading part takes about 1600ms which is now off the main thread. Pretty neat!

Referencing issues Closes #191