SPARK-1829 Sub-second durations shouldn't round to "0 s"

As "99 ms" up to 99 ms
As "0.1 s" from 0.1 s up to 0.9 s

https://issues.apache.org/jira/browse/SPARK-1829

Compare the first image to the second here: http://imgur.com/RaLEsSZ,7VTlgfo#0

Author: Andrew Ash <andrew@andrewash.com>

Closes #768 from ash211/spark-1829 and squashes the following commits:

1c15b8e [Andrew Ash] SPARK-1829 Format sub-second durations more appropriately

(cherry picked from commit a3315d7f4c7584dae2ee0aa33c6ec9e97b229b48)
Signed-off-by: Reynold Xin <rxin@apache.org>