Added stream operator for filter::ip::PortRange.

Review: https://reviews.apache.org/r/21505
MESOS-1376: Fixed an invalid reference in the Registrar.
Added backoff to slave's initial (re-)registration attempt.

Review: https://reviews.apache.org/r/21464
Set work_dir flag for mesos-local.

Review: https://reviews.apache.org/r/21562
Fixed invalid comment string in slave.cpp
Set slave subsystems only in containerizer tests and cleaned up the
code using cgroups::enabled().

Review: https://reviews.apache.org/r/21585
Included the error message when nl_connect fails.

Review: https://reviews.apache.org/r/21595
Added elected time to master's web ui.

Review: https://reviews.apache.org/r/21501
Fixed the broken build on OSX caused by cgroups::enabled().

Review: https://reviews.apache.org/r/21664
Minor fix to pre-commit hook.
Updated post-reviews to work with 'post-review' (RBTools < 0.6) and
'rbt' (RBtools >= 0.6).

Review: https://reviews.apache.org/r/21257
Add os::getgid() and os::getuid() using reentrant version of getpwnam.
Also, updated os::user() to use reentrant version of getpwuid.

Review: https://reviews.apache.org/r/20955
Add ExecEnv for use with execle to safely exec.

execle() should be used with envp to set a child's environment variables
in an async safe manner.

Review: https://reviews.apache.org/r/20956
Update Subprocess to use ExecEnv from stout.

Review: https://reviews.apache.org/r/20957
Make execute in MesosContainerizer async signal safe.

Test MesosContainerizer redirects task stdout and stderr.

Review: https://reviews.apache.org/r/20958
Cleaned up some style issues in MesosContainerizer
Correct include of stout/os/execenv.hpp in stout distribution
Added file permissions to open() call when O_CREAT is specified.

Review: https://reviews.apache.org/r/21778
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