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
Updated the master to handle status update acknowledgement messages.

Review: https://reviews.apache.org/r/21838
Added metrics for acknowledgement messages in the master.

Review: https://reviews.apache.org/r/21839
Fixed MesosContainerizerExecuteTest.IoRedirection for OSX.

Review: https://reviews.apache.org/r/21833
Added error message to system metrics failures.

Review: https://reviews.apache.org/r/21989
Updated the slave to send terminal tasks for terminated executors
during re-registration.

The slave must send terminal unacknowledged tasks for terminal
executors so that the master can correctly reconcile tasks when
the slave re-registers.

Otherwise, the master may see a missing task
(terminal, unacknowledged) on the slave and incorrectly inform
the framework via TASK_LOST, per MESOS-1388.

Review: https://reviews.apache.org/r/21991
Fixed master to properly rescind offers.

Review: https://reviews.apache.org/r/21948
Fixed Master::launchTasks() to inform allocator of unused resources
when any of the offers are invalid.

Review: https://reviews.apache.org/r/21750
Fixed master to remove and rescind offers when a slave is
disconnected.

Review: https://reviews.apache.org/r/21961
Introduced io::redirect.

Review: https://reviews.apache.org/r/21998