Don't use application logger

Separate logger for extension is better and much more flexible, e.g. you can
define custom handlers or easy reuse existing. For example, snippet to reuse all the
handlers of the application:

```python
for handler in app.logger.handlers:
    apify.logger.addHandler(handler)
```

- [x] Create a default logger
- [x] Add example of handler configuration
- [x] Test suite