Cancel background tasks

Like [someone asked](https://groups.google.com/forum/#!topic/androidannotations/1_T8_YN2hj0) on the mailing list, I think it would be useful to be able to cancel background tasks, typically in `onStop()`:

~~~java
    @Override
    protected void onStop() {
        super.onStop();
        BackgroundExecutor.cancelAll("longtask");
    }

    @Background(id="longtask")
    public void doSomethingLong() {
        // ...
    }
~~~

I implemented it over my changes adding the [background execution serialization](https://github.com/excilys/androidannotations/pull/564) feature (I changed a bit the implementation).

`@Background` now supports `id` (used for cancellation), `delay` (was already implemented) and `serial` (for serializing tasks) attributes, and any association of them.

As the background execution feature must be reliable, and the interactions between serializing and cancelling must behave correctly, I wrote a set of functional tests.