Add Python API.

core/src/main/scala/spark/api/python/PythonRDD.scala

pyspark/pyspark/__init__.py

pyspark/pyspark/context.py

pyspark/pyspark/examples/__init__.py

pyspark/pyspark/examples/kmeans.py

pyspark/pyspark/examples/pi.py

pyspark/pyspark/examples/tc.py

pyspark/pyspark/java_gateway.py

pyspark/pyspark/join.py

pyspark/pyspark/rdd.py

pyspark/pyspark/serializers.py

pyspark/pyspark/worker.py

pyspark/requirements.txt

python/tc.py