SyslogUdpHandler: Send a valid rfc5424 version

I had some problems getting my log messages parsed correctly by Papertrail. As far as I can see from [this guide](http://help.papertrailapp.com/kb/configuration/configuring-remote-syslog-from-embedded-or-proprietary-systems/), the current header format of SyslogUdpHandler is neither valid [rfc5424](https://tools.ietf.org/html/rfc5424) nor [rfc3164](http://www.ietf.org/rfc/rfc3164.txt) (I had a quick look at the standards themselves too, but I haven't studied them thouroughly). 

I also made the class protected rather than private, so it can be overridden in case anyone wants to use RFC 3164 instead.