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.
It seems like you can eliminate most of the cases in this `unpack` method since most of them don't need special handling.  It looks like three distinct cases are Records, which we want to unpack into Maps, arrays, in which we want to recursively unpack any Records, and primitive types, which we pass through unchanged.