[SPARK-1519] Support minPartitions param of wholeTextFiles() in PySpark

Author: Kan Zhang <kzhang@apache.org>

Closes #697 from kanzhang/SPARK-1519 and squashes the following commits:

4f8d1ed [Kan Zhang] [SPARK-1519] Support minPartitions param of wholeTextFiles() in PySpark

(cherry picked from commit f18fd05b513b136363c94adb3e5b841f8bf48134)
Signed-off-by: Reynold Xin <rxin@apache.org>