Move logstash formatting logic out of logstash encoder

This allows it to be reused in other components.
Obvious layout implementation using logstash formatter
Implement socket appender on top of the syslog implementation

It turns out that the syslog base is not syslog-specific, but just ships
the data in any given layout over UDP. (The facilty / severity are not
used in the default implementation.)

If you configure logstash like so:

    input {
      udp {
        port => 5515
        format => json_event
        tag => my_tag_for_logback_events
      }
    }

and logback like so:

    <appender name="LOGSTASH" class="net.logstash.logback.appender.LogstashSocketAppender">
      <host>my.logstash.server.local</host>
      <port>5515</port>
    </appender>

log messages will be sent directly to the server, with no need for a
logstash daemon running locally.
Merge pull request #11 from bkirwi/push-logs

Push logs
Update contributors
I hate Maven sometimes
Grr
If a job's worth doing, don't let Maven near it
Update version to 1.7-SNAPSHOT after release of 1.6
Going to have to try an old version of Maven
Ugh
Try upgrading the maven-scm-plugin
Grumpy now
Fling more at it
[maven-release-plugin] prepare release logstash-logback-encoder-1.9
[maven-release-plugin] prepare for next development iteration
emit version 1 of logstash json format, see LOGSTASH-675 for a description
update to latest available version
adapt documentation to new format
Merge pull request #13 from heymjo/master

implement post logstash 1.2 JSON format