add support for rfc5424 in SyslogAppender

This is similar to http://jira.qos.ch/browse/LOGBACK-588 but simpler. 

Supports in message prefix:
1. milliseconds in timestamp
2. appName field
3. messageId field
4. structured data: specifying an id triggers using MDC data for key/values in structured data

Documentation has been updated (SyslogAppender section).