SPARK-1284: Fix improper use of SimpleDateFormat

`SimpleDateFormat` is not thread-safe. Some places use the same SimpleDateFormat object without safeguard in the multiple threads. It will cause that the Web UI displays improper date.

This PR creates a new `SimpleDateFormat` every time when it's necessary. Another solution is using `ThreadLocal` to store a `SimpleDateFormat` in each thread. If this PR impacts the performance, I can change to the latter one.

Author: zsxwing <zsxwing@gmail.com>

Closes #179 from zsxwing/SPARK-1278 and squashes the following commits:

21fabd3 [zsxwing] SPARK-1278: Fix improper use of SimpleDateFormat

Conflicts:

	core/src/main/scala/org/apache/spark/scheduler/JobLogger.scala
	core/src/main/scala/org/apache/spark/ui/jobs/JobProgressUI.scala
	core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala
	core/src/main/scala/org/apache/spark/ui/jobs/StageTable.scala
	core/src/main/scala/org/apache/spark/util/FileLogger.scala
Fix to Stage UI to display numbers on progress bar

Fixes an issue on Stage UI to display numbers on progress bar which are today hidden behind the progress bar div. Please refer to the attached images to see the issue.
![screen shot 2014-03-21 at 4 48 46 pm](https://f.cloud.github.com/assets/563652/2489083/8c127e80-b153-11e3-807c-048ebd45104b.png)
![screen shot 2014-03-21 at 4 49 00 pm](https://f.cloud.github.com/assets/563652/2489084/8c12cf5c-b153-11e3-8747-9d93ff6fceb4.png)

Author: Emtiaz Ahmed <emtiazahmed@gmail.com>

Closes #201 from emtiazahmed/master and squashes the following commits:

a7964fe [Emtiaz Ahmed] Fix to Stage UI to display numbers on progress bar

(cherry picked from commit 646e55405b433fdedc9601dab91f99832b641f87)
Signed-off-by: Aaron Davidson <aaron@databricks.com>