[SPARK-2373]RDD add span function (split an RDD to two RDD based on user's function)]

Hi all,
This function is a basic function in Scala.
def span(p: T => Boolean): (RDD[T], RDD[T]) 
Splits this RDD into a prefix/suffix pair according to a predicate .
returns
a pair consisting of the longest prefix of this RDD whose elements all satisfy p, and the rest of this list.

JIRA:https://issues.apache.org/jira/browse/SPARK-2373
This needs scaladoc, since this method's meaning won't be clear to users unless they're familiar with the Scala `span` function.