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 think we can remove the propertyPrivate. Like I said before, I really expect anyone who is configuring this to know what they're doing :)