tests so we don't get the test spark.conf on the classpath.

The test in context.py created two different instances of the
SparkContext class by copying "globals", so that some tests can have a
global "sc" object and others can try initializing their own contexts.
This led to two JVM gateways being created since SparkConf also looked
at pyspark.context.SparkContext to get the JVM.

Also remove takePartition from PythonRDD and use collectPartition in rdd.py.

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().

If we support custom serializers, the Python
worker will know what type of input to expect,
so we won't need to wrap Tuple2 and Strings into
pickled tuples and strings.

The constructor for SparkContext should pass in self so that we track
the current context and produce errors if another one is created. Add
a doctest to make sure creating multiple contexts triggers the
exception.

Add a new classmethod to SparkContext to set system properties like is
possible in Scala/Java. Unlike the Java/Scala implementations, there's
no access to System until the JVM bridge is created. Since
SparkContext handles that, move the initialization of the JVM
connection to a separate classmethod that can safely be called
repeatedly as long as the same instance (or no instance) is provided.

The sc.StorageLevel -> StorageLevel pathway is a bit janky, but otherwise
the shell would have to call a private method of SparkContext. Having
StorageLevel available in sc also doesn't seem like the end of the world.
There may be a better solution, though.

As for creating the StorageLevel object itself, this seems to be the best
way in Python 2 for creating singleton, enum-like objects:
http://stackoverflow.com/questions/36932/how-can-i-represent-an-enum-in-python

It uses reflection... I am not proud of that fact, but it at least ensures
compatibility (sans refactoring of the StorageLevel stuff).

* RDD, *RDDFunctions -> org.apache.spark.rdd
* Utils, ClosureCleaner, SizeEstimator -> org.apache.spark.util
* JavaSerializer, KryoSerializer -> org.apache.spark.serializer

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

One unfortunate consequence of this fix is that we materialize any
collections that are given to us as generators, but this seems necessary
to get reasonable behavior on small collections. We could add a
batchSize parameter later to bypass auto-computation of batch size if
this becomes a problem (e.g. if users really want to parallelize big
generators nicely)

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.

This should avoid exceptions caused by existing
files with different contents.

I also removed some unused code.