Author: Reynold Xin <rxin@databricks.com>

Closes #4386 from rxin/df-implicits and squashes the following commits:

9d96606 [Reynold Xin] style fix
edd296b [Reynold Xin] ReplSuite
1c946ab [Reynold Xin] [SPARK-5612][SQL] Move DataFrame implicit functions into SQLContext.implicits.