Fixed flaky FaultToleranceTest.ReconcileIncompleteTasks test.

Review: https://reviews.apache.org/r/12064
Fixed flaky MasterDetectorExpireSlaveZKSessionNewMaster test.

Review: https://reviews.apache.org/r/12065
Fixed slave to create work directory during initialization.

Review: https://reviews.apache.org/r/12010
Added a log message in the master for tasks lost during slave removal.

Review: https://reviews.apache.org/r/12044
Improved the configure output when deprecated headers are present
on OS X.

Review: https://reviews.apache.org/r/12045
Added Jenkins scheduler.

Review: https://reviews.apache.org/r/12033
Added Jenkins plugin to the build tool chain.

Review: https://reviews.apache.org/r/12034
Fixed links in webui for command executor tasks.

Some frameworks don't report an executor ID, but have a task ID. As a
workaround, we can use the task ID when the executor ID is empty.

Review: https://reviews.apache.org/r/11814
Added committers and a java compiler target to mesos.pom.in.

Review: https://reviews.apache.org/r/12087
Killed os::alive because its semantics was somewhat confusing and
caused some flaky tests.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/12073
Updated the Reaper to use os::process instead of os::alive.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/12074
Fixed bad Java comparison (use equals(), not ==).

Review: https://reviews.apache.org/r/11488
Fixed slave to validate slave id during (re-)registration.

Review: https://reviews.apache.org/r/12122
Added missing os/exists.hpp to un-break the build.