[SPARK-2871] [PySpark] add approx API for RDD

RDD.countApprox(self, timeout, confidence=0.95)

        :: Experimental ::
        Approximate version of count() that returns a potentially incomplete
        result within a timeout, even if not all tasks have finished.

        >>> rdd = sc.parallelize(range(1000), 10)
        >>> rdd.countApprox(1000, 1.0)
        1000

RDD.sumApprox(self, timeout, confidence=0.95)

        Approximate operation to return the sum within a timeout
        or meet the confidence.

        >>> rdd = sc.parallelize(range(1000), 10)
        >>> r = sum(xrange(1000))
        >>> (rdd.sumApprox(1000) - r) / r < 0.05

RDD.meanApprox(self, timeout, confidence=0.95)

        :: Experimental ::
        Approximate operation to return the mean within a timeout
        or meet the confidence.

        >>> rdd = sc.parallelize(range(1000), 10)
        >>> r = sum(xrange(1000)) / 1000.0
        >>> (rdd.meanApprox(1000) - r) / r < 0.05
        True

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

Closes #2095 from davies/approx and squashes the following commits:

e8c252b [Davies Liu] add approx API for RDD