Improve memory usage when the worker promise is used many times.

[hirohitokato]

I improved the following points in the code:

• The initial event listener calls reject every time the worker posts a message. (b1913d3)
• The event listeners for worker promise remains even if the task has finished. (9237ae5, 6d025e9, da7bdd1)

And I also changed example(app.js at 28fdebd) so that we can understand that the worker promise is easily reusable. (Ideally, I should prepare 3 workerPromise for each "Execute!" button...)

I would appreciate it if you read my code. 😃

[hirohitokato]

Of course, I don't change the APIs & behaviors for users.

[FredrikSandell]

Looks good. Appreciate you taking the time to improve the code!

[hirohitokato]

Thanks a lot!!