Merge pull request #502 from pwendell/clone-1

Remove Hadoop object cloning and warn users making Hadoop RDD's.

The code introduced in #359 used Hadoop's WritableUtils.clone() to
duplicate objects when reading from Hadoop files. Some users have
reported exceptions when cloning data in various file formats,
including Avro and another custom format.

This patch removes that functionality to ensure stability for the
0.9 release. Instead, it puts a clear warning in the documentation
that copying may be necessary for Hadoop data sets.
(cherry picked from commit c3196171f3dffde6c9e67e3d35c398a01fbba846)

Conflicts:

	core/src/main/scala/org/apache/spark/api/java/JavaSparkContext.scala
Merge pull request #503 from pwendell/master

Fix bug on read-side of external sort when using Snappy.

This case wasn't handled correctly and this patch fixes it.
(cherry picked from commit 3d6e75419330d27435becfdf8cfb0b6d20d56cf8)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>