Add events for Flower

[gersh]

Add "task-*" events so we can monitor the queue with Flower.

[clokep]

Any chance that tests could be added for this?

[gersh]

Is there a good way to test if a signal has been sent with pytest? I tried:

    class TaskMonitor:
        def __init__(self):
            self.tasks_received = 0
            self.tasks_started = 0
            self.tasks_suceeded = 0

        def on_task_received(self, event):
            self.tasks_received += 1
        def on_task_started(self, event):
            self.tasks_started += 1
        def on_task_suceeded(self, event):
            self.tasks_suceeded += 1

    tm = TaskMonitor()

    recv = celery_app.events.Receiver(connection, handlers={
            'task-received': tm.on_task_received,
            'task-started': tm.on_task_started,
            'task-suceeded': tm.on_task_suceeded,
            '*': tm.on_other
            })

but I can't seem to get it to work. If you can help me figure out how to get it to listen for events in a pytest, I can implement it. I tested it in Flower, and it seems to work, but I'm not sure how to get the internals to work for a unit test.

[clokep]

Some of the tests use the SignalCounter class to do this (it is a bit hacky, but I haven't managed to figure out a better way to do it).

[clokep]

See also #70.