Use ILogger and ILoggerFactory instead of explicit calls to NLog

[wopietrz]

It is not possible to use any other logging framework than NLog right now. The logger should be injected using standard ILogger and ILoggerFactory interfaces. That would remove the strong dependency between NLog and Cybersource Rest Client.

Example of coupling below:

    private static Logger logger;
    private ExceptionFactory _exceptionFactory = (name, response) => null;

    ...
    public KeyGenerationApi(string basePath)
    {
        ...

        if (logger == null)
        {
            logger = LogManager.GetCurrentClassLogger();
        }
    }

[xantari]

Related to #15 that they closed a few years back...