Add an add() method to pyspark accumulators.

Add a regular method for adding a term to accumulators in
pyspark. Currently if you have a non-global accumulator, adding to it
is awkward. The += operator can't be used for non-global accumulators
captured via closure because it's involves an assignment. The only way
to do it is using __iadd__ directly.

Adding this method lets you write code like this:

def main():
    sc = SparkContext()
    accum = sc.accumulator(0)

    rdd = sc.parallelize([1,2,3])
    def f(x):
        accum.add(x)
    rdd.foreach(f)
    print accum.value

where using accum += x instead would have caused UnboundLocalError
exceptions in workers. Currently it would have to be written as
accum.__iadd__(x).