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.
Unbundled picojson

Enable mesos to build against the system installed version of picojson.

Review: https://reviews.apache.org/r/23091
Unbundled libev

Enable mesos to build against the system installed version of libev.

Review: https://reviews.apache.org/r/23414
Send an unregister message when the slave is shutting itself down.

Review: https://reviews.apache.org/r/22367
Added metrics for port mapping network isolator.

Review: https://reviews.apache.org/r/23492
Fix executor passing STDIN pipe to health check.

Previous fix for STDIN only fixes the health check command not passing
in STDIN pipe by reading file STDIN. However the health check command
is still getting STDIN piped as the command executor is creating a
PIPE to STDIN when launching health check. To really fix this we need
to also fix not passing anything to STDIN when launching health check.

Review: https://reviews.apache.org/r/23518
Reenable health check tests
Allow overridable launcher dir