Expose the logdev

[oggy]

The motivation here is to fix a Rails issue caused by the way ActiveSupport extends the ruby Logger to add broadcasting of messages to multiple destinations. I believe the problem could be much more elegantly solved if Logger exposed the underlying LogDevice. Going by repo history, the existence of this object hasn't changed since 2003, so I think it's stable enough to expose.

In addition to letting you read the logdev, this also lets you pass a logdev in. To implement broadcasting, we could now define a LogDevice subclass that delegates its 3 methods to the LogDevices of a list of underlying loggers, and simply create a new logger with this device.

What do you think?