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