[SPARK-5138][SQL] Ensure schema can be inferred from a namedtuple

When attempting to infer the schema of an RDD that contains namedtuples, pyspark fails to identify the records as namedtuples, resulting in it raising an error.

Example:

```python
from pyspark import SparkContext
from pyspark.sql import SQLContext
from collections import namedtuple
import os

sc = SparkContext()
rdd = sc.textFile(os.path.join(os.getenv('SPARK_HOME'), 'README.md'))
TextLine = namedtuple('TextLine', 'line length')
tuple_rdd = rdd.map(lambda l: TextLine(line=l, length=len(l)))
tuple_rdd.take(5)  # This works

sqlc = SQLContext(sc)

# The following line raises an error
schema_rdd = sqlc.inferSchema(tuple_rdd)
```

The error raised is:
```
  File "/opt/spark-1.2.0-bin-hadoop2.4/python/pyspark/worker.py", line 107, in main
    process()
  File "/opt/spark-1.2.0-bin-hadoop2.4/python/pyspark/worker.py", line 98, in process
    serializer.dump_stream(func(split_index, iterator), outfile)
  File "/opt/spark-1.2.0-bin-hadoop2.4/python/pyspark/serializers.py", line 227, in dump_stream
    vs = list(itertools.islice(iterator, batch))
  File "/opt/spark-1.2.0-bin-hadoop2.4/python/pyspark/rdd.py", line 1107, in takeUpToNumLeft
    yield next(iterator)
  File "/opt/spark-1.2.0-bin-hadoop2.4/python/pyspark/sql.py", line 816, in convert_struct
    raise ValueError("unexpected tuple: %s" % obj)
TypeError: not all arguments converted during string formatting
```

Author: Gabe Mulley <gabe@edx.org>

Closes #3978 from mulby/inferschema-namedtuple and squashes the following commits:

98c61cc [Gabe Mulley] Ensure exception message is populated correctly
375d96b [Gabe Mulley] Ensure schema can be inferred from a namedtuple