Add JsonStringMessageDecoder from camus' wikimedia branch

The file's source is
  camus-etl-kafka/src/main/java/com/linkedin/camus/etl/kafka/coders/JsonStringMessageDecoder.java
in cea32b3500a4698ecaf3be3ea728ffc9f0f579b5 of the wikimedia branch of
  https://github.com/wikimedia/camus.git

Tests, etc are in follow-up commits

Change-Id: I6999be423f2791c029ca889dd2880a2467424b97