[SPARK-4477] [PySpark] remove numpy from RDDSampler

In RDDSampler, it try use numpy to gain better performance for possion(), but the number of call of random() is only (1+faction) * N in the pure python implementation of possion(), so there is no much performance gain from numpy.

numpy is not a dependent of pyspark, so it maybe introduce some problem, such as there is no numpy installed in slaves, but only installed master, as reported in SPARK-927.

It also complicate the code a lot, so we may should remove numpy from RDDSampler.

closes #2313

Note: this patch including some commits that not mirrored to github, it will be OK after it catches up.