Typo: and -> an

Author: Andrew Ash <andrew@andrewash.com>

Closes #927 from ash211/patch-5 and squashes the following commits:

79b577d [Andrew Ash] Typo: and -> an
updated link to mailing list

Author: nchammas <nicholas.chammas@gmail.com>

Closes #923 from nchammas/patch-1 and squashes the following commits:

65c4d18 [nchammas] updated link to mailing list
SPARK-1976: fix the misleading part in  streaming docs

Spark streaming requires at least two working threads, but the document gives the example like

import org.apache.spark.api.java.function._
import org.apache.spark.streaming._
import org.apache.spark.streaming.api._
// Create a StreamingContext with a local master
val ssc = new StreamingContext("local", "NetworkWordCount", Seconds(1))
http://spark.apache.org/docs/latest/streaming-programming-guide.html

Author: CodingCat <zhunansjtu@gmail.com>

Closes #924 from CodingCat/master and squashes the following commits:

bb89f20 [CodingCat] update streaming docs
[SPARK-1959] String "NULL" shouldn't be interpreted as null value

JIRA issue: [SPARK-1959](https://issues.apache.org/jira/browse/SPARK-1959)

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #909 from liancheng/spark-1959 and squashes the following commits:

306659c [Cheng Lian] [SPARK-1959] String "NULL" shouldn't be interpreted as null value