Support forwarding container logs to syslog

Docker has feature to view container logs (output to stdout and stderr),
but it is not very convenient. This patch adds an option to enable
forwarding of log messages to syslog server.

To enable syslog forwarding, add "--syslog" to docker's "run" command's
arguments. It will forward messages to local (host's) syslog server. To
forward to remote syslog, use "--syslog-url=udp://server:port"
parameter.

Other options allow configuring syslog parameters:
* --syslog-facility
* --syslog-severity
* --syslog-tag
it's failing here http://golang.org/pkg/log/syslog/#Priority, http://golang.org/src/pkg/log/syslog/syslog.go?s=823:840#L21 

The docker image used by drone is still trying to build windows, which this does not support, so this will be an issue.