Adding StringRecordWriterProvider.

The returned RecordWriter will write out the CamusRecord's data as String bytes.
This is useful for writing to HDFS directly what comes out of Kafka, instead of
always having to use Avro.
Adding configuration for etl.output.record.delimiter in StringRecordWriterProvider.
Merge pull request #28 from wikimedia/camus-kafka-0.8-pull-requests

Adding StringRecordWriterProvider.