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 went back and forth on it being a text field versus a drop down list. What I really want is a combo box with an open text field that can be populated with a dropdown, but I can't have everything. On one hand, the logback SyslogAppender implementation will reject an invalid facility, but there are also facilities that are technically valid but would probably cause problems if someone selected one of them, like KERN, or AUTH, or SYSLOG. I didn't want to allow people who don't know what they are doing to screw things up by sending AEM log messages to facilities that look relevant to what they are logging but are instead dedicated to processes running at the OS level. I also think the USER facility is going to be preferred for most use cases, and you'd only really want to use a LOCAL# when you know it's not being used by something else and you need it separated for some reason.

Maybe I should just make it propertyPrivate so that value can only be changed from a sling:OsgiConfig node.