Added three configuration properties to SyslogAppender

Added three configuration properties to SyslogAppender: facility, stacktrace.pattern, and throwable.excluded. These provide more flexibility for segregation of log output for different audiences and systems, such as for creating audit-only logs, structured logs, and logs intended for debugging application errors.
I agree. I put it there because it was something I specifically ran into when trying to get syslog setup on my mac and only found it at the end of the Logback documentation on the SyslogAppender. I wish it was front-and-center when I started down this path, so I just stuck it right there on the Felix config panel for anyone else like me who regularly fails to RTFM. 

How would I go about making changes to the docs? Should I just make pull requests for the gh-pages-1.6 branch? If so, I'll remove that statement from the metatype description and move it to the Syslog Appender page.