systemd support

- added support for systemd. If `platform==fedora` and `platform_version >= 15`, this will be used.
- added support for setting specific `uid` and `gid` on the logstash user for orgs that allocate specific IDs. leave nil to let system pick (default)
- made the `server.log_file` attribute optional. If it's set to empty string, logstash will be configured to log to stdout instead of a file. this is useful for systemd boxes which log stdout from services into journald. also useful for runit
. Not so useful for init.d.