Exposed the authenticated principal through Authenticator::authenticate() result.

Review: https://reviews.apache.org/r/21665
Advanced clock to ensure GarbageCollector prune runs in test.

Review: https://reviews.apache.org/r/21782
Unauthorized HTTP Response + tests.

Review: https://reviews.apache.org/r/21324
Fix inline base64 functions.

Review: https://reviews.apache.org/r/21796
Added Owned::release().

Review: https://reviews.apache.org/r/21721
Added TemporaryDirectoryTest fixture to stout tests.

Review: https://reviews.apache.org/r/21590
Added JSON support to flags.

Review: https://reviews.apache.org/r/21565
Fixed pointer ownership bug in Authorizer.

Review: https://reviews.apache.org/r/21727
Fixed an incorrect configuration option.
Updated flags in the configuration documentation.
Integrate Authorizer into Master.

Review: https://reviews.apache.org/r/21723
Fixed the use of a boolean in the master's stats.json endpoint.
Added retry for zookeeper_init per MESOS-1326.

Often during temporary DNS failovers or outages, we see slaves
aborting in zookeeper_init.

In many cases, the slave can restart within 10 seconds. Since
retrying zookeeper_init should be safe to do, this is an attempt
to minimize the number of unnecessary aborts in the slave.

Review: https://reviews.apache.org/r/21783
Reduced acknowledgment delay in the scheduler driver.

We currently 'defer' in order to acknowledge processed status
updates in the scheduler driver.

This is unfortunate because it adds unnecessary delay and
unnecessary retries, especially when the scheduler's queue
is backed up.

It is sufficient to use 'aborted' to see if we can send a status
update. Ideally there would be proper locking on this variable
rather than the use of 'volatile', but since we're mostly concerned
about the driver aborting on the same thread as the status update
was processed, I've left it as is.

Review: https://reviews.apache.org/r/21788
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