Update NLogSink.cs

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.
Merge pull request #242 from nslowes/patch-1

Update NLogSink to map Verbose level to Trace level in NLog