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.
Merge branch 'camus-kafka-0.8-pull-requests' of github.com:wikimedia/camus into camus-kafka-0.8-pull-requests

Conflicts:
	camus-etl-kafka/src/main/java/com/linkedin/camus/etl/kafka/coders/JsonStringMessageDecoder.java