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
Enriched the Java Docs in the src/java files.

Review: https://reviews.apache.org/r/21080
Checked IP header length for IP packet filters.

Review: https://reviews.apache.org/r/23525
Fixed a flaky test.

Review: https://reviews.apache.org/r/23543
Added a low level scheduler example using pthread.

Review: https://reviews.apache.org/r/23216
Changed healthy status to be sent when task killed after health check.

Currently after the health check receives configured amount of task
fails it will initiate a kill task. However currently the task kill
status update doesn't set the healthy field so it's unclear was it
killed because health check or other reasons. This patch sets the health
check field when the task is killed because of failing health checks.

Review: https://reviews.apache.org/r/23520
Fixed variable naming in a test.
Renamed slaves/frameworks activated/deactivated

The existing terminology is confusing both for "slaves.deactivated" and
"frameworks.activated". Currently a deactivated slave actually
represents a removed/shutdown slave and "frameworks.activated" map holds
both activated and deactivated frameworks.
In order to make things look clear, rename the following:
* master.slaves.deactivated -> master.slaves.removed
* master.slaves.activated -> master.slaves.registered
* master.frameworks.activated -> master.frameworks.registered
* allocator.slaveDisconnect -> allocator.slaveDeactivate
* allocator.slaveReconnected -> allocator.slaveReactivated

Review: https://reviews.apache.org/r/23147
Re-enabled health check tests

Currently the health check tests are disabled as it was flaky. Part of the reason is that all assertions was EXPECTs instead of ASSERTs so it will continue to execute.
Another issue was it's currently discovering the path to launch health check executable from the command executor main argv[0] path.
However, the correct path is to launch the generated script that does library path resolution, which is the parent of .libs.

Review: https://reviews.apache.org/r/23443
Fixed a deadlock bug in Mutex.

Review: https://reviews.apache.org/r/23592
Disabled flaky low level scheduler tests.