Push logs

It's sometimes more convenient to push logging events to the server, instead of having a logstash instance running locally and tailing logs. This pull includes a `LogstashSocketAppender` that pushes logstash-friendly events out over UDP.

The implementation delegates most of the work to the base syslog appender, though this requires pulling the formatting logic out so it can be reused in a new `LogstashLayout`.

Example logstash config:

    input {
      udp {
        port => 5515
        format => json_event
        tag => my_tag_for_logback_events
      }
    }

And in logback.xml, you'd have something like:

    <appender name="LOGSTASH" class="net.logstash.logback.appender.LogstashSocketAppender">
      <host>my.logstash.server.local</host>
      <port>5515</port>
    </appender>