[SPARK-2010] [PySpark] [SQL] support nested structure in SchemaRDD (part 1)

Convert Row in JavaSchemaRDD into Array[Any] and unpickle them as tuple in Python, then convert them into namedtuple, so use can access fields just like attributes.

This will let nested structure can be accessed as object, also it will reduce the size of serialized data and better performance.

root
 |-- field1: integer (nullable = true)
 |-- field2: string (nullable = true)
 |-- field3: struct (nullable = true)
 |    |-- field4: integer (nullable = true)
 |    |-- field5: array (nullable = true)
 |    |    |-- element: integer (containsNull = false)
 |-- field6: array (nullable = true)
 |    |-- element: struct (containsNull = false)
 |    |    |-- field7: string (nullable = true)

Then we can access them by row.field3.field5[0]  or row.field6[5].field7

`Row` can not be accessed directly here, only by self.__class__, maybe self._FIELDS will be better? 

btw, namedtuple uses `_fields` for names, namedtuple does not support name startswith "_", so we can not use namedtuple?