add java class 'JavaWordCountProducer' to 'provide java word count producer'.

1.run example of streaming kafka,currently missing java word count producer,not conducive to java developers to learn and test.
add a class JavaKafkaWordCountProducer.

2.run example of JavaKafkaWordCount.I find no java word count producer.
run example of KafkaWordCount.I find have scala word count producer.
I think we should provide the corresponding example code to facilitate java developers to learn and test.

3.My project team develops spark applications,basically with java statements and java API.