[SPARK-4397] Move object RDD to the front of RDD.scala.

I ran into multiple cases that SBT/Scala compiler was confused by the implicits in continuous compilation mode. Adding explicit return types fixes the problem.

Author: Reynold Xin <rxin@databricks.com>

Closes #3580 from rxin/rdd-implicit and squashes the following commits:

ee32fcd [Reynold Xin] Move object RDD to the end of the file.
b8562c9 [Reynold Xin] Merge branch 'master' of github.com:apache/spark into rdd-implicit
d4e9f85 [Reynold Xin] Code review.
a836a37 [Reynold Xin] Move object RDD to the front of RDD.scala.
[SPARK-4421] Wrong link in spark-standalone.html

Modified the link of building Spark.

Author: Masayoshi TSUZUKI <tsudukim@oss.nttdata.co.jp>

Closes #3279 from tsudukim/feature/SPARK-4421 and squashes the following commits:

56e31c1 [Masayoshi TSUZUKI] Modified the link of building Spark.
Fix typo in Spark SQL docs.

Author: Andy Konwinski <andykonwinski@gmail.com>

Closes #3611 from andyk/patch-3 and squashes the following commits:

7bab333 [Andy Konwinski] Fix typo in Spark SQL docs.