[SPARK-3094] [PySpark] compatitable with PyPy

After this patch, we can run PySpark in PyPy (testing with PyPy 2.3.1 in Mac 10.9), for example:

```
PYSPARK_PYTHON=pypy ./bin/spark-submit wordcount.py
```

The performance speed up will depend on work load (from 20% to 3000%). Here are some benchmarks:

 Job | CPython 2.7 | PyPy 2.3.1  | Speed up
 ------- | ------------ | ------------- | -------
 Word Count | 41s   | 15s  | 2.7x
 Sort | 46s |  44s | 1.05x
 Stats | 174s | 3.6s | 48x

Here is the code used for benchmark:

```python
rdd = sc.textFile("text")
def wordcount():
    rdd.flatMap(lambda x:x.split('/'))\
        .map(lambda x:(x,1)).reduceByKey(lambda x,y:x+y).collectAsMap()
def sort():
    rdd.sortBy(lambda x:x, 1).count()
def stats():
    sc.parallelize(range(1024), 20).flatMap(lambda x: xrange(5024)).stats()
```

Author: Davies Liu <davies.liu@gmail.com>

Closes #2144 from davies/pypy and squashes the following commits:

9aed6c5 [Davies Liu] use protocol 2 in CloudPickle
4bc1f04 [Davies Liu] refactor
b20ab3a [Davies Liu] pickle sys.stdout and stderr in portable way
3ca2351 [Davies Liu] Merge branch 'master' into pypy
fae8b19 [Davies Liu] improve attrgetter, add tests
591f830 [Davies Liu] try to run tests with PyPy in run-tests
c8d62ba [Davies Liu] cleanup
f651fd0 [Davies Liu] fix tests using array with PyPy
1b98fb3 [Davies Liu] serialize itemgetter/attrgetter in portable ways
3c1dbfe [Davies Liu] Merge branch 'master' into pypy
42fb5fa [Davies Liu] Merge branch 'master' into pypy
cb2d724 [Davies Liu] fix tests
9986692 [Davies Liu] Merge branch 'master' into pypy
25b4ca7 [Davies Liu] support PyPy