- Add charset support to the json codec. While technically JSON is
required to be UTF-8, some systems generate JSON that is not UTF-8
encoded. One observed example is nxlog on windows using this nxlog
config:

    <Output out>
      Module om_tcp
      Host 10.1.1.2
      Port 3515
      Exec $EventReceivedTime = integer($EventReceivedTime) / 1000000; \
      to_json();
    </Output>

The JSON emitted with nxlog above will generate JSON encoded with
Latin-1/CP1252/Windows-1252. This is incorrect, but logstash can easily
work around this now.
- fix logstash-event gemspec