[HOTFIX] Patching maven build after #6 (SPARK-1121).

That patch removed the Maven avro declaration but didn't remove the
actual dependency in core. /cc @scrapcodes

Author: Patrick Wendell <pwendell@gmail.com>

Closes #37 from pwendell/master and squashes the following commits:

0ef3008 [Patrick Wendell] [HOTFIX] Patching maven build after #6 (SPARK-1121).
Removed reference to incubation in Spark user docs.

Author: Reynold Xin <rxin@apache.org>

Closes #2 from rxin/docs and squashes the following commits:

08bbd5f [Reynold Xin] Removed reference to incubation in Spark user docs.
Remote BlockFetchTracker trait

This trait seems to have been created a while ago when there
were multiple implementations; now that there's just one, I think it
makes sense to merge it into the BlockFetcherIterator trait.

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #39 from kayousterhout/remove_tracker and squashes the following commits:

8173939 [Kay Ousterhout] Remote BlockFetchTracker.
SPARK-1032. If Yarn app fails before registering, app master stays aroun...

...d long after

This reopens https://github.com/apache/incubator-spark/pull/648 against the new repo.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #28 from sryza/sandy-spark-1032 and squashes the following commits:

5953f50 [Sandy Ryza] SPARK-1032. If Yarn app fails before registering, app master stays around long after
SPARK-1051. On YARN, executors don't doAs submitting user

This reopens https://github.com/apache/incubator-spark/pull/538 against the new repo

Author: Sandy Ryza <sandy@cloudera.com>

Closes #29 from sryza/sandy-spark-1051 and squashes the following commits:

708ce49 [Sandy Ryza] SPARK-1051. doAs submitting user in YARN