Add a Syslog octet counting frame decoder. (RFC6587)

See: http://tools.ietf.org/html/rfc6587#section-3.4.1
This could link to http://www.graylog2.org/resources/documentation/sending/syslog but we'd need to add a section about syslog-ng and maybe a section on how to create a Syslog Input in Graylog2 to that document.