[SPARK-6027][SPARK-5546] Fixed --jar and --packages not working for KafkaUtils...

[SPARK-6027][SPARK-5546] Fixed --jar and --packages not working for KafkaUtils and improved error message

The problem with SPARK-6027 in short is that JARs like the kafka-assembly.jar does not work in python as the added JAR is not visible in the classloader used by Py4J. Py4J uses Class.forName(), which does not uses the systemclassloader, but the JARs are only visible in the Thread's contextclassloader. So this back uses the context class loader to create the KafkaUtils dstream object. This works for both cases where the Kafka libraries are added with --jars spark-streaming-kafka-assembly.jar or with --packages spark-streaming-kafka

Also improves the error message.

davies

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #4779 from tdas/kafka-python-fix and squashes the following commits:

fb16b04 [Tathagata Das] Removed import
c1fdf35 [Tathagata Das] Fixed long line and improved documentation
7b88be8 [Tathagata Das] Fixed --jar not working for KafkaUtils and improved error message