Refactor of interpreter demo
* Renamed demos/interpreter/index.html as demos/interpreter/step-execution.html (including redirect), and added demos/interpreter/async-execution.html.
* Refactored code to automatically generate/parse the blocks, eliminating the need for a "Parse JavaScript" button. Code is still shown in alert upon stepping to the first statement. Print statements now write to output <textarea> instead of modal dialogs.
