[SPARK-4413][SQL] Parquet support through datasource API

Goals:
 - Support for accessing parquet using SQL but not requiring Hive (thus allowing support of parquet tables with decimal columns)
 - Support for folder based partitioning with automatic discovery of available partitions
 - Caching of file metadata

See scaladoc of `ParquetRelation2` for more details.

Author: Michael Armbrust <michael@databricks.com>

Closes #3269 from marmbrus/newParquet and squashes the following commits:

1dd75f1 [Michael Armbrust] Pass all paths for FileInputFormat at once.
645768b [Michael Armbrust] Review comments.
abd8e2f [Michael Armbrust] Alternative implementation of parquet based on the datasources API.
938019e [Michael Armbrust] Add an experimental interface to data sources that exposes catalyst expressions.
e9d2641 [Michael Armbrust] logging / formatting improvements.

(cherry picked from commit 02ec058efe24348cdd3691b55942e6f0ef138732)
Signed-off-by: Michael Armbrust <michael@databricks.com>
add Sphinx as a dependency of building docs

Author: Davies Liu <davies@databricks.com>

Closes #3388 from davies/doc_readme and squashes the following commits:

daa1482 [Davies Liu] add Sphinx dependency

(cherry picked from commit 8cd6eea6298fc8e811dece38c2875e94ff863948)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>
[SPARK-4522][SQL] Parse schema with missing metadata.

This is just a quick fix for 1.2.  SPARK-4523 describes a more complete solution.

Author: Michael Armbrust <michael@databricks.com>

Closes #3392 from marmbrus/parquetMetadata and squashes the following commits:

bcc6626 [Michael Armbrust] Parse schema with missing metadata.

(cherry picked from commit 90a6a46bd11030672597f015dd443d954107123a)
Signed-off-by: Michael Armbrust <michael@databricks.com>
[SPARK-4472][Shell] Print "Spark context available as sc." only when SparkContext is created...

... successfully

It's weird that printing "Spark context available as sc" when creating SparkContext unsuccessfully.

Author: zsxwing <zsxwing@gmail.com>

Closes #3341 from zsxwing/SPARK-4472 and squashes the following commits:

4850093 [zsxwing] Print "Spark context available as sc." only when SparkContext is created successfully

(cherry picked from commit f1069b84b82b932751604bc20d5c2e451d57c455)
Signed-off-by: Reynold Xin <rxin@databricks.com>