Make sure seattlerb/syslogger includes released 2.0 code

I recently installed SyslogLogger 2.0, and was super confused when there were:

1. difference in API from seattlerb/syslogger
2. no idea where 2.0 was released from

Eventually tracked it down to @tenderlove's repo. This is mostly his changes, but I also updated README.txt to refer to the correct constant, and updated History.txt with the release date of 2.0.

*Edit:* accidentally some english