Add intergration seam to Logger

[KevM]

In our usage of websocket-sharp we need to integrate our logging infrastructure with websocket-sharp's. This PR adds a seam which allows the Logger implementation to be swapped out. Existing users are not affected.

Downstream users wishing to use their own logger would simply use the Log setter:

var webSocket = new WebSocket("ws://websocket.org/echo")
{
  Log = new Log4NetWebSocketLogger()
};

Example integration

Below is an integration example for plain Log4Net.

public class Log4NetWebSocketLogger : IWebSocketLogger
{
    private readonly ILog _log;

    public Log4NetWebSocketLogger()
    {
        _log = LogManager.GetLogger(typeof(WebSocket));
    }

    public void Debug(string message)
    {
        _log.Debug(message);
    }

    public void Error(string message)
    {
        _log.Error(message);
    }

    public void Fatal(string message)
    {
        _log.Fatal(message);
    }

    public void Info(string message)
    {
        _log.Info(message);
    }

    public void Trace(string message)
    {
        _log.Debug(message);
    }

    public void Warn(string message)
    {
        _log.Warn(message);
    }
}

Thank you for your consideration. Let me know if there is anything I can do to improve this PR.

[sta]

Hello there, thanks for your pull request.

I would like to merge this fix, but how does this change the log level (, set the file output, and change the output behavior)?

[KevM]

Hello back, It does not change the log level or any existing behavior. This PR simply exposes a seam for users to change out the built in logger with their own.