Add basic support for converting Avro records to JSON for output

 * refactoring and fixes of the original PR
 * added key_error, payload_error optional fields to JSON message
   envelope to indicate deserialization errors.
 * prepare for future Avro producer when we (or preferably avro-c)
   figure out how to convert JSON to Avro.
 * fix librdkafka version in bootstrap.sh
 * make Dockerfile rely more on bootstrap.sh
 * use edenhill fork of yajl to allow embedding the avro-to-json
   deserialized message key/value directly in the JSON envelope with -J.

Merge branch 'output_avro_as_json' of github.com:llofberg/kafkacat into llofberg-output_avro_as_json

Requires libavro and libserdes to compile.