Added 'metadata' to the replicated log to support log recovery (and
likely in the future log reconfiguration).

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/15799
Allow HTTP requests to a slave omit "slave(1)".

Review: https://reviews.apache.org/r/15798
Fixed the ContenderDetectorShutdownNetwork test flakiness.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/15845
Drop messages from non-leading Masters in the scheduler driver.

Review: https://reviews.apache.org/r/15778
Added build instructions for Mac OS X Mavericks.

From: Niklas Nielsen <nik@qni.dk>
Review: https://reviews.apache.org/r/15727
Fixed master to ignore messages from unregistered framework.

Review: https://reviews.apache.org/r/15773
Reduced the minimum number of cpus per offer to 0.1.

Review: https://reviews.apache.org/r/15774
Fixed status update manager to backoff on status update retries.

Review: https://reviews.apache.org/r/15851