MESOS-511 Update documentation home and remove outdated drafts pages.
Added upgrade instructions for 0.15.0.
Updated CHANGELOG for 0.15.0.
Updated version to 0.16.0.
Fixed Makefile to properly refer to README.md.
Renames filenames of documentation to be lowercase and updates some documentation templates for consistency.
Updates link to Spark getting started, and removes linkering textile formatting.
Made Promise have a virtual destructor.

See summary.

From: Benjamin Hindman <benh@berkeley.edu>
Review: https://reviews.apache.org/r/14382
Added the registrar.

From: Benjamin Hindman <benh@berkeley.edu>
Review: https://reviews.apache.org/r/14383
Injected Registrar into the Master and the tests.

From: Benjamin Hindman <benh@berkeley.edu>
Review: https://reviews.apache.org/r/14382
Fixed a null pointer dereference in LevelDBStorage::initialize.
Redirected navigation to framework page if slave was gone.

If an invalid slave ID is used in the redirect controller, redirect the
request to either the homepage or the frameworks page.

Fixes MESOS-650

Review: http://reviews.apache.org/r/15144
Supported upgrading from Shared to Owned.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/15112
Fixed race condition that caused Authenticatee process to deadlock.

Review: https://reviews.apache.org/r/15138
Fix for building with gcc 4.7.

Review: https://reviews.apache.org/r/15177
Replace api.github.com for mark-down conversion.

Using api.github.com for mark-down rendering limits off-line use and cause trouble with help is excessively used on a shared IP due to api quota rules.

This patch includes Christopher Jeffrey's Marked mark-down compiler (https://github.com/chjj/marked) and replaces the github api call.

From: Niklas Nielsen <nik@qni.dk>
Review: https://reviews.apache.org/r/14940
Fixed FaultToleranceTest.SchedulerFailoverFrameworkMessage test.

Review: https://reviews.apache.org/r/15191
Changed mesos-ps so that it runs on python2.6 or above.

From: Shingo Omura <everpeace@gmail.com>
Review: https://reviews.apache.org/r/14941
Modified mesos-execute so that it reports the framework id and task id.

From: Shingo Omura <everpeace@gmail.com>
Review: https://reviews.apache.org/r/15161
Updated CHANGELOG for 0.15.0-rc3.