Add a Syslog octet counting frame decoder. (RFC6587)

See: http://tools.ietf.org/html/rfc6587#section-3.4.1
While it's rather unlikely to happen here, [`Character#isDigit(char)`](http://docs.oracle.com/javase/7/docs/api/java/lang/Character.html#isDigit(char)) matches much more than just [0-9]. Maybe rather use `return '0' <= message.getByte(0) && message.getByte(0) <= '9';` instead.