Merge pull request #528 from mengxr/sample. Closes #528.

 Refactor RDD sampling and add randomSplit to RDD (update)

Replace SampledRDD by PartitionwiseSampledRDD, which accepts a RandomSampler instance as input. The current sample with/without replacement can be easily integrated via BernoulliSampler and PoissonSampler. The benefits are:

1) RDD.randomSplit is implemented in the same way, related to https://github.com/apache/incubator-spark/pull/513
2) Stratified sampling and importance sampling can be implemented in the same manner as well.

Unit tests are included for samplers and RDD.randomSplit.

This should performance better than my previous request where the BernoulliSampler creates many Iterator instances:
https://github.com/apache/incubator-spark/pull/513

Author: Xiangrui Meng <meng@databricks.com>

== Merge branch commits ==

commit e8ce957e5f0a600f2dec057924f4a2ca6adba373
Author: Xiangrui Meng <meng@databricks.com>
Date:   Mon Feb 3 12:21:08 2014 -0800

    more docs to PartitionwiseSampledRDD

commit fbb4586d0478ff638b24bce95f75ff06f713d43b
Author: Xiangrui Meng <meng@databricks.com>
Date:   Mon Feb 3 00:44:23 2014 -0800

    move XORShiftRandom to util.random and use it in BernoulliSampler

commit 987456b0ee8612fd4f73cb8c40967112dc3c4c2d
Author: Xiangrui Meng <meng@databricks.com>
Date:   Sat Feb 1 11:06:59 2014 -0800

    relax assertions in SortingSuite because the RangePartitioner has large variance in this case

commit 3690aae416b2dc9b2f9ba32efa465ba7948477f4
Author: Xiangrui Meng <meng@databricks.com>
Date:   Sat Feb 1 09:56:28 2014 -0800

    test split ratio of RDD.randomSplit

commit 8a410bc933a60c4d63852606f8bbc812e416d6ae
Author: Xiangrui Meng <meng@databricks.com>
Date:   Sat Feb 1 09:25:22 2014 -0800

    add a test to ensure seed distribution and minor style update

commit ce7e866f674c30ab48a9ceb09da846d5362ab4b6
Author: Xiangrui Meng <meng@databricks.com>
Date:   Fri Jan 31 18:06:22 2014 -0800

    minor style change

commit 750912b4d77596ed807d361347bd2b7e3b9b7a74
Author: Xiangrui Meng <meng@databricks.com>
Date:   Fri Jan 31 18:04:54 2014 -0800

    fix some long lines

commit c446a25c38d81db02821f7f194b0ce5ab4ed7ff5
Author: Xiangrui Meng <meng@databricks.com>
Date:   Fri Jan 31 17:59:59 2014 -0800

    add complement to BernoulliSampler and minor style changes

commit dbe2bc2bd888a7bdccb127ee6595840274499403
Author: Xiangrui Meng <meng@databricks.com>
Date:   Fri Jan 31 17:45:08 2014 -0800

    switch to partition-wise sampling for better performance

commit a1fca5232308feb369339eac67864c787455bb23
Merge: ac712e4 cf6128f
Author: Xiangrui Meng <meng@databricks.com>
Date:   Fri Jan 31 16:33:09 2014 -0800

    Merge branch 'sample' of github.com:mengxr/incubator-spark into sample

commit cf6128fb672e8c589615adbd3eaa3cbdb72bd461
Author: Xiangrui Meng <meng@databricks.com>
Date:   Sun Jan 26 14:40:07 2014 -0800

    set SampledRDD deprecated in 1.0

commit f430f847c3df91a3894687c513f23f823f77c255
Author: Xiangrui Meng <meng@databricks.com>
Date:   Sun Jan 26 14:38:59 2014 -0800

    update code style

commit a8b5e2021a9204e318c80a44d00c5c495f1befb6
Author: Xiangrui Meng <meng@databricks.com>
Date:   Sun Jan 26 12:56:27 2014 -0800

    move package random to util.random

commit ab0fa2c4965033737a9e3a9bf0a59cbb0df6a6f5
Author: Xiangrui Meng <meng@databricks.com>
Date:   Sun Jan 26 12:50:35 2014 -0800

    add Apache headers and update code style

commit 985609fe1a55655ad11966e05a93c18c138a403d
Author: Xiangrui Meng <meng@databricks.com>
Date:   Sun Jan 26 11:49:25 2014 -0800

    add new lines

commit b21bddf29850a2c006a868869b8f91960a029322
Author: Xiangrui Meng <meng@databricks.com>
Date:   Sun Jan 26 11:46:35 2014 -0800

    move samplers to random.IndependentRandomSampler and add tests

commit c02dacb4a941618e434cefc129c002915db08be6
Author: Xiangrui Meng <meng@databricks.com>
Date:   Sat Jan 25 15:20:24 2014 -0800

    add RandomSampler

commit 8ff7ba3c5cf1fc338c29ae8b5fa06c222640e89c
Author: Xiangrui Meng <meng@databricks.com>
Date:   Fri Jan 24 13:23:22 2014 -0800

    init impl of IndependentlySampledRDD