Fixed health check command subprocess handling.

Currently it doesn't check all subprocess future states, and also
creates a pipe to stdin which can cause some scripts to expect stdin
input and blocks.

Review: https://reviews.apache.org/r/23416
Add Sailthru to the "Powered by Mesos" page.

This closes: #21
Review: https://github.com/apache/mesos/pull/21
Added CHANGELOG for 0.19.1.
Updated the upgrades document for 0.18.2 and 0.19.1.
Added logging to vote.sh to help debug a confusing exit.