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.
Merge branch '0.7.0' of https://github.com/getpantheon-cookbooks/chef-logstash into getpantheon-cookbooks-0.7.0

* extended to agent

Conflicts:
	attributes/default.rb
	templates/default/logstash_server.conf.erb
	templates/default/sv-logstash_server-run.erb
fix syntax error
add systemd service unit for logstash_server
add additional systemd service files
Merge pull request #224 from getpantheon-cookbooks/fedora_systemd

add logstash server and agent systemd unit files
switch to fedora 18,  chef-zero repo to own recipe
fixes to zeromq packages
Merge branch 'fedora_systemd'

Conflicts:
	CHANGELOG.md
	Vagrantfile
	attributes/agent.rb
	attributes/server.rb
	metadata.rb
	recipes/agent.rb
	recipes/server.rb
	templates/default/logstash_server.conf.erb