Fixed ExecutorInfo handover on the ExternalContainerizer launch.

see summary.

Review: https://reviews.apache.org/r/21683
Fixed ExternalContainerizer Subprocess use for non sandboxed invocations.

When using Subprocess on non sandboxed invocations of the external
containerizer program, stderr was not getting redirected at all,
possibly triggering deadlocks due to a filled stderr pipe that was
not getting drained.
This patch redirects to /dev/null when no sandbox is given for an
invocation. See MESOS-1336.

Review: https://reviews.apache.org/r/21675
Fixed concurrent destroy invocations on the ExternalContainerizer.

Review: https://reviews.apache.org/r/21680
Added support for s3/s3n to the mesos fetcher.

Review: https://reviews.apache.org/r/21852
Fix for web UI redirect message not showing up.

There was a bug that caused the "We're redirecting you to the leader"
message to not show up when visiting the web UI on a follower. Looks
to me like it was caused by an upgrade in twitter bootstrap, either
way it's a simple fix. I also changed the style from error to warning,
since the red message seemed a little harsh.

Review: https://reviews.apache.org/r/21850
Fixed compiler errors in routing library when an old glibc is used.

Review: https://reviews.apache.org/r/21822
Returned false in link::remove() if NLE_NODEV is received.

Review: https://reviews.apache.org/r/21823
Fixed the flaky SlaveRecovery MultipleFrameworks test.

Review: https://reviews.apache.org/r/21871
Fixed ExternalContainerizer wait behavior when getting destroyed.

When 'wait' was terminated by 'destroy', it is getting SIGKILLed. We
need to test for that specific case and setup the appropriate
Termination message accordingly as it can not get delivered by the
external containerizer program itself.

Review: https://reviews.apache.org/r/21681
Updates Mesos version in Getting Started guide.
Added workaround to allow the ExternalContainerizer containers to get recovered via slave state.

The slave currently expects a forked executor PID within its states
when recovering. For the ExternalContainerizer however this does not
make sense but is introduced in this patch to get around the problem
as described in MESOS-1328 and MESOS-923.

Review: https://reviews.apache.org/r/21677
Fixed blank line in flags/parse.hpp.
Keep track of the principals for authenticated pids in Master and verify FramworkInfo::principal during (re-)registration.

Review: https://reviews.apache.org/r/21787
Adds URX to #PoweredByMesos page.
Timer now uses Clock instead of Stopwatch to enable easier testing.

Review: https://reviews.apache.org/r/20918
Exposed event queue size from SchedulerProcess.

Review: https://reviews.apache.org/r/21558
Ported libprocess system/stats.json endpoint to new metrics library.

Review: https://reviews.apache.org/r/19545
Updated metric names for consistency.

Review: https://reviews.apache.org/r/21720
Uninlined large function from usage.hpp.

Review: https://reviews.apache.org/r/21829
Updated the slave to handle status update acknowledgements from the master.

Review: https://reviews.apache.org/r/21837