Update NLogSink to map Verbose level to Trace level in NLog

Mapped Verbose level messages to Trace in NLog.  They are both the "lowest" or most verbose log level so they seem to map naturally: https://github.com/NLog/NLog/wiki/Log-levels.  If this was omitted intentionally I would be interested in the reasoning.  Also corrected the default log message which indicated log4net instead of NLog.