Example pyspark-inputformat for Avro file format

This is an example showing how to map an Avro file to a pyspark RDD.

Starting Pyspark with
`SPARK_CLASSPATH=examples/target/scala-2.10/spark-examples-1.1.0-SNAPSHOT-hadoop1.0.4.jar IPYTHON=1 bin/pyspark`

    avroRdd = sc.newAPIHadoopFile("/tmp/data.avro", 
      "org.apache.avro.mapreduce.AvroKeyInputFormat", 
      "org.apache.avro.mapred.AvroKey", 
      "org.apache.hadoop.io.NullWritable",
      keyConverter="org.apache.spark.examples.pythonconverters.AvroGenericConverter")

Note that not all data types are implemented.

Note also that null values from UNION(NULL, ...) fields will show up as the empty String "" in python.
How about adding ```case null => null``` for null handling?