Fixed GroupTest.MultipleGroups test.

Review: https://reviews.apache.org/r/14961
MESOS-511 Update documentation home and remove outdated drafts pages.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