Add support for syslog formatting and TLS

With this commit seelog can be used to log to syslog, optionally over TLS (e.g. to rsyslog) so logs can be transmitted directly to a central log server.

Below is an example configuration logging over a TLS connection, using syslog facility local4 (=20).
```xml
<outputs formatid="syslog">
  <filter levels="info,warn,error,critical">
    <conn net="tcp4" addr="server.address:5514" useTLS="true" insecureSkipVerify="true" />
  </filter>
</outputs>
<formats>
  <format id="syslog" format="&lt;%PRI(20)&gt;1 %Date(2006-01-02T15:04:05Z07:00) myhostname myappname PROCID MSGID - %Msg%n"/>
</formats>
```
Use "-" for the PROCID and MSGID if they are unknown, refer to http://tools.ietf.org/html/rfc5424 for more information about the format.