This PR updates spark-submit to allow submitting Python scripts (currently only with deploy-mode=client, but that's all that was supported before) and updates the PySpark code to properly find various paths, etc. One significant change is that we assume we can always find the Python files either from the Spark assembly JAR (which will happen with the Maven assembly build in make-distribution.sh) or from SPARK_HOME (which will exist in local mode even if you use sbt assembly, and should be enough for testing). This means we no longer need a weird hack to modify the environment for YARN.

This patch also updates the Python worker manager to run python with -u, which means unbuffered output (send it to our logs right away instead of waiting a while after stuff was written); this should simplify debugging.

In addition, it fixes https://issues.apache.org/jira/browse/SPARK-1709, setting the main class from a JAR's Main-Class attribute if not specified by the user, and fixes a few help strings and style issues in spark-submit.

In the future we may want to make the `pyspark` shell use spark-submit as well, but it seems unnecessary for 1.0.

Author: Matei Zaharia <matei@databricks.com>

Closes #664 from mateiz/py-submit and squashes the following commits:

15e9669 [Matei Zaharia] Fix some uses of path.separator property
051278c [Matei Zaharia] Small style fixes
0afe886 [Matei Zaharia] Add license headers
4650412 [Matei Zaharia] Add pyFiles to PYTHONPATH in executors, remove old YARN stuff, add tests
15f8e1e [Matei Zaharia] Set PYTHONPATH in PythonWorkerFactory in case it wasn't set from outside
47c0655 [Matei Zaharia] More work to make spark-submit work with Python:
d4375bd [Matei Zaharia] Clean up description of spark-submit args a bit and add Python ones

This reopens https://github.com/apache/incubator-spark/pull/640 against the new repo

Author: Sandy Ryza <sandy@cloudera.com>

Closes #30 from sryza/sandy-spark-1004 and squashes the following commits:

89889d4 [Sandy Ryza] Move unzipping py4j to the generate-resources phase so that it gets included in the jar the first time
5165a02 [Sandy Ryza] Fix docs
fd0df79 [Sandy Ryza] PySpark on YARN

Fixes SPARK-970.

For now, this only adds MarshalSerializer, but it lays the groundwork
for other supporting custom serializers.  Many of these mechanisms
can also be used to support deserialization of different data formats
sent by Java, such as data encoded by MsgPack.

This also fixes a bug in SparkContext.union().

Implementing SPARK-878 for PySpark: adding zip and egg files to context and passing it down to workers which add these to their sys.path

The problem was that the gateway was being initialized whenever the
pyspark.context module was loaded.  The fix uses lazy initialization
that occurs only when SparkContext instances are actually constructed.

I also made the gateway and jvm variables private.

This change results in ~3-4x performance improvement when running the
PySpark unit tests.

Fix minor documentation formatting issues.