Rename top-level 'pyspark' directory to 'python'

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

docs/_plugins/copy_api_dirs.rb

pyspark-shell

python/.gitignore

python/epydoc.conf

python/examples/kmeans.py

python/examples/logistic_regression.py

python/examples/pi.py

python/examples/transitive_closure.py

python/examples/wordcount.py

python/lib/PY4J_LICENSE.txt

python/lib/PY4J_VERSION.txt

python/lib/py4j0.7.egg

python/lib/py4j0.7.jar

python/pyspark/__init__.py

python/pyspark/broadcast.py

python/pyspark/cloudpickle.py

python/pyspark/context.py

python/pyspark/java_gateway.py

python/pyspark/join.py