Fixed hard tabs in monitor.cpp

From: Niklas Nielsen <nik@qni.dk>
Review: https://reviews.apache.org/r/15500
Fixed string leaks in os.hpp.

From: Niklas Nielsen <nik@qni.dk>
Review: https://reviews.apache.org/r/15368
Added list of TaskStatuses to Task message.

Task now includes repeated task status with timestamps.  This permits
caltulating the start/finish time for tasks (among other things).

To support this, a timestamp was added to the TaskStatus message which
is also passed to frameworks when there are state transitions.

Review: https://reviews.apache.org/r/14434
Normalized task start/finish time in Web UI.

Review: https://reviews.apache.org/r/15539
Added terminated frameworks to frameworks tab.

In preparation for moving frameworks off the homepage, terminated
frameworks will now be listed on /frameworks.html.

The code was copy/pasted from home.html.

Review: http://reviews.apache.org/r/15526
Moved offers to their own tab and page.

In preparation for moving Tasks to the homepage, Offers moved to their
own page and tab. They are available as top-level navigation, and the
homepage table is now gone.

Review: http://reviews.apache.org/r/15527
Fixed an import error in futures.py.

When queue is empty, the name Queue.Empty must be imported.

From: Du Li <lidu008@gmail.com>
Review: https://reviews.apache.org/r/15585
Fixed the flaky pstree test.
Offer an execvp like interface for running tasks.

Review: https://reviews.apache.org/r/15542