Consistently invoke bash with /usr/bin/env bash in scripts to make code more portable (JIRA Ticket SPARK-817)

The previous version assumed that a CLASSPATH environment variable was
set by the "run" script when launching the process that starts the
ExecutorRunner, but unfortunately this is not true in tests. Instead, we
factor the classpath calculation into an extenral script and call that.

NOTE: This includes a Windows version but hasn't yet been tested there.

check for core classes in run. This fixed the problem that core tests
depend on whether repl module is compiled or not.

1) Add support for HADOOP_CONF_DIR (and/or YARN_CONF_DIR - use either) : which is used to specify the client side configuration directory : which needs to be part of the CLASSPATH.

2) Move from var+=".." to var="$var.." : the former does not work on older bash shells unfortunately.

Reversed the order of tests to find a scala executable (in the case when SPARK_LAUNCH_WITH_SCALA is defined): instead of checking in the PATH first, and only then (if not found) for SCALA_HOME, now we check for SCALA_HOME first, and only then (if not defined) do we look in the PATH.  The advantage is that now if the user has a more recent (non-compatible) version of scala in her PATH, she can use SCALA_HOME to point to the older (compatible) version for use with spark.

Suggested by Josh Rosen in this thread:

  https://groups.google.com/forum/?fromgroups=#!topic/spark-users/NC9JKvP8808

See the JIRA for more details.

I was only able to test the bash version (don't have Windows)
so maybe check over that the syntax is correct there.

- we do not need getEnvOrEmpty
- Instead of saving SPARK_NONDAEMON_JAVA_OPTS, it would be better to modify the scripts to use a different variable name for the JAVA_OPTS they do eventually use

Conflicts:
	docs/_config.yml

- Bundle Py4J binaries, since it's hard to install
- Uses Spark's `run` script to launch the Py4J
  gateway, inheriting the settings in spark-env.sh

With these changes, (hopefully) nothing more than
running `sbt/sbt package` will be necessary to run
PySpark.

Fixed bugs in RawNetworkInputDStream and in its examples. Made the ReducedWindowedDStream persist RDDs to MEMOERY_SER_ONLY by default. Removed unncessary examples. Added streaming-env.sh.template to add recommended setting for streaming.

with Mesos being in Maven