Add a shorthand

With this pull request, people won't have to type in `doInBackground`, they can just run:

```javascript
var task = new AsyncTask((a, b) -> a + b)
```

It also works with options:

```javascript
var task = new AsyncTask((a, b) -> a + b, {keepAlive: true})
```

This pull request also comes with a new test for the short hand stuff, and an update to the `README.md` which mentions this shorthand API.

Also, this is 100% backwards compatible. And it passes tests.