[SPARK-927] detect numpy at time of use

it is possible for numpy to be installed on the driver node but not on
worker nodes. in such a case, using the rddsampler's constructor to
detect numpy leads to failures on workers as they cannot import numpy
(see below).

the solution here is to detect numpy right before it is used on the
workers.

example code & error -

yum install -y numpy
pyspark
>>> sc.parallelize(range(10000)).sample(False, .1).collect()
...
14/09/07 10:50:01 WARN TaskSetManager: Lost task 4.0 in stage 0.0 (TID 4, node4): org.apache.spark.api.python.PythonException: Traceback (most recent call last):
  File "/home/test/spark/dist/python/pyspark/worker.py", line 79, in main
    serializer.dump_stream(func(split_index, iterator), outfile)
  File "/home/test/spark/dist/python/pyspark/serializers.py", line 196, in dump_stream
    self.serializer.dump_stream(self._batched(iterator), stream)
  File "/home/test/spark/dist/python/pyspark/serializers.py", line 127, in dump_stream
    for obj in iterator:
  File "/home/test/spark/dist/python/pyspark/serializers.py", line 185, in _batched
    for item in iterator:
  File "/home/test/spark/dist/python/pyspark/rddsampler.py", line 115, in func
    if self.getUniformSample(split) <= self._fraction:
  File "/home/test/spark/dist/python/pyspark/rddsampler.py", line 57, in getUniformSample
    self.initRandomGenerator(split)
  File "/home/test/spark/dist/python/pyspark/rddsampler.py", line 42, in initRandomGenerator
    import numpy
ImportError: No module named numpy
        org.apache.spark.api.python.PythonRDD$$anon$1.read(PythonRDD.scala:124)
        org.apache.spark.api.python.PythonRDD$$anon$1.<init>(PythonRDD.scala:154)
        org.apache.spark.api.python.PythonRDD.compute(PythonRDD.scala:87)
        org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:262)
        org.apache.spark.rdd.RDD.iterator(RDD.scala:229)
        org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:62)
        org.apache.spark.scheduler.Task.run(Task.scala:54)
        org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:177)
        java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        java.lang.Thread.run(Thread.java:744)