Use different Hadoop version for YARN artifacts.

This uses a seperate Hadoop version for YARN artifact. This means when people link against
spark-yarn, things will resolve correctly.
Merge pull request #929 from pwendell/master

Use different Hadoop version for YARN artifacts.
Explain yarn.version in Maven build docs
Bumping Mesos version to 0.13.0
Merge pull request #931 from pwendell/yarn-docs

Explain yarn.version in Maven build docs
Merge pull request #932 from pwendell/mesos-version

Bumping Mesos version to 0.13.0
Fix typo in Maven build docs
Merge pull request #933 from jey/yarn-typo-fix

Fix typo in Maven build docs
Fix issue with spark_ec2 seeing empty security groups

Under unknown, but occasional, circumstances, reservation.groups is empty
despite reservation.instances each having groups. This means that the
spark_ec2 get_existing_clusters() method would fail to find any instances.
To fix it, we simply use the instances' groups as the source of truth.

Note that this is actually just a revival of PR #827, now that the issue
has been reproduced.
Merge pull request #938 from ilikerps/master

Fix issue with spark_ec2 seeing empty security groups
Simr stuff
new strategy for launching
Avoid getting an exception on all Spark examples when Spark home isn't set and Seq(null) is passed to SC.
No more hardcoded filesnames.
refactoring with error messages
added config to change number of cores for executors in SIMR
added config to change number of cores for executors in SIMR
Avoiding system exit
stop() driver instead of exit() in the end of example.
Added functionality to stop executors.