[SPARK-2736] PySpark converter and example script for reading Avro files

JIRA: https://issues.apache.org/jira/browse/SPARK-2736

This patch includes:
1. An Avro converter that converts Avro data types to Python. It handles all 3 Avro data mappings (Generic, Specific and Reflect).
2. An example Python script for reading Avro files using AvroKeyInputFormat and the converter.
3. Fixing a classloading issue.

cc @MLnick @JoshRosen @mateiz

Author: Kan Zhang <kzhang@apache.org>

Closes #1916 from kanzhang/SPARK-2736 and squashes the following commits:

02443f8 [Kan Zhang] [SPARK-2736] Adding .avsc files to .rat-excludes
f74e9a9 [Kan Zhang] [SPARK-2736] nit: clazz -> className
82cc505 [Kan Zhang] [SPARK-2736] Update data sample
0be7761 [Kan Zhang] [SPARK-2736] Example pyspark script and data files
c8e5881 [Kan Zhang] [SPARK-2736] Trying to work with all 3 Avro data models
2271a5b [Kan Zhang] [SPARK-2736] Using the right class loader to find Avro classes
536876b [Kan Zhang] [SPARK-2736] Adding Avro to Java converter

(cherry picked from commit 9422a9b084e3fd5b2b9be2752013588adfb430d0)
Signed-off-by: Matei Zaharia <matei@databricks.com>