Included mesos-cat and mesos-tail in installation.

Review: https://reviews.apache.org/r/15299
Created 'usage' and moved 'get' helper to cli.py.

Review: https://reviews.apache.org/r/15300
Refactored mesos-cat to use helpers from cli.py.

Review: https://reviews.apache.org/r/15301
Revised os::realpath to return Result for when path does not exist.

Review: https://reviews.apache.org/r/15302
Refactored mesos-ps to display CPU and memory information.

From: Shingo Omura <everpeace@gmail.com>
Review: https://reviews.apache.org/r/15200
Refactored mesos-tail to use helpers from cli.py.

From: Shingo Omura <everpeace@gmail.com>
Review: https://reviews.apache.org/r/15336
Added a flash service for UI alerts after redirects.

When an error occurs during a redirect, it is useful to show an error
message on the following page view rather than the current one. In
particular, when clicking a sandbox link for a slave that is
inaccessible, the `SlaveExecutorRerouterCtrl` sends a request to the
desired slave and redirects back to root if an error occurs. It's not
possible to show an error during the redirect, and so rendering on
the root view is ideal.

Flash alerts are rendered once and then cleaned up; they will not
display more than once.

Review: http://reviews.apache.org/r/15198
Expanded stout::format() to handle 11 parameters.

From: Eric Biederman <ebiederm@xmission.com>
Review: https://reviews.apache.org/r/15013
Improved cgroups isolator to report additional memory statistics.

From: Eric Biederman <ebiederm@xmission.com>
Review: https://reviews.apache.org/r/15015
Fixed typos and errors in the Log Javadoc.
Added a note about the reliability of kill task requests.
Fixed typos in the Log javadoc.
Updated CHANGELOG for 0.15.0-rc4.
Updates powered by mesos list to include Devicescape, HubSpot, Vimeo, Pinkbike, and OpenTable.
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