Ensure an Airbrake logger doesn't have multiple AirbrakeHandlers.

Rather than creating new loggers each time `airbrake.getLogger` is called, check if one has already been configured.
[`logging.getLogger` will always return you the same logger when given the same name](https://docs.python.org/2/library/logging.html#logging.getLogger), so I don't think we need to keep track of loggers we have seen in `LOGGERS`. 

What if we just check the logger object we are returning for the handler?

instead of
```python
logger = logging.getLogger(name)
ab = AirbrakeHandler(**kwargs)
logger.addHandler(ab)
```

You could do:

```python
logger = logging.getLogger(name)
if not any([isinstance(handler, AirbrakeHandler)
            for handler in logger.handlers]):
    ab = AirbrakeHandler(**kwargs)
    logger.addHandler(ab)
```