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.
One thing you could try to avoid NullPointerException is to wrap any intermediate ```value``` that could be null into ```Option(value)``` and only at the end of transformation call ```.orNull``` on it to get the value.