Adding JsonStringMessageDecoder.java to look for timestamp fields in JSON string payloads.

To use, set:

```
  camus.message.decoder.class=com.linkedin.camus.etl.kafka.coders.JsonStringMessageDecoder
  camus.message.timestamp.format=<your-timestamp-format:HH:mm:ss>
  camus.message.timestamp.field=<your-timestamp-field>
```

This adds ```com.google.code.gson``` as a dependency.