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().
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.
