add sample for async system implementation

[jorinvo]

The API of the emulator is built with Promises to allow an asynchronous implementation.
The current implementation is all synchronous, in the browser.
All the commands are designed in a way to only interact with the state through the "system calls".
This way it should be possible to implement the "system calls" asynchronous without changing the commands.

An asynchronous implementation could run the system calls on a server for example, persist them or even allow multiple users to interact with the same system this way.

Even better would be to think first about further decoupling the system from the index.js file. Would be nice to be able to choose with which system to initialize the emulator instance.