Supported O_CLOEXEC for os::open on all platforms.

Review: https://reviews.apache.org/r/26715
Added a protobuf::append function.

Review: https://reviews.apache.org/r/26799
Added a protobuf::append test.

Review: https://reviews.apache.org/r/26800
Added the missing --path to the replicated log advisory message.
Fixed verify-reviews.py to not apply submitted reviews.

Review: https://reviews.apache.org/r/26827
 Temporarily disabled Module tests due to MESOS-1942.
Escaped JSON object keys.

All strings in JSON need to be escaped, including the keys in
dictionaries / objects.

Review: https://reviews.apache.org/r/26723
Refactored stout os open/close/fcntl.

Review: https://reviews.apache.org/r/26843
Updated verify_reviews.py to use the git pre-commit hook.

Review: https://reviews.apache.org/r/26798
Renamed verify-reviews.py to verify_reviews.py.
Fixed default port values for master and slave in the documentation.

Review: https://reviews.apache.org/r/26254
Fixed a file descriptor leak in StatusUpdateManager.

Review: https://reviews.apache.org/r/26879
Fixed MESOS-947: Slave should properly handle a killTask() that arrives
between runTask() and _runTask().
Temporarily disabled KillTaskBetweenRunTaskPart test due to MESOS-1945.