Enabled toggling between relative and absolute timestamps by clicking on them.

Note that your preference for which type you'd like is remembered
between sessions.

Issue: https://issues.apache.org/jira/browse/MESOS-789

Review: http://reviews.apache.org/r/16712

From: Thomas Rampelberg <thomas@saunter.org>
Review: https://reviews.apache.org/r/16712
Showed start/stop time for tasks of completed frameworks.

From: Thomas Rampelberg <thomas@saunter.org>
Review: https://reviews.apache.org/r/16715
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