Moved flags to stout.

Review: https://reviews.apache.org/r/11465
Replaced flags and configurator in Mesos with flags in stout.

Review: https://reviews.apache.org/r/11466
Added a 'Logging' process to libprocess.

Review: https://reviews.apache.org/r/11467
Removed logging process from Mesos (now in libprocess).

Review: https://reviews.apache.org/r/11468
Updated libprocess to use '3rdparty' instead of 'third_party'.

Review: https://reviews.apache.org/r/11469
Renamed 'third_party' to '3rdparty'.

Review: https://reviews.apache.org/r/11470
Added stout specific 'CHECK' constructs.

Review: https://reviews.apache.org/r/11472
Replaced Mesos CHECK_SOME with stout CHECK_SOME.

Review: https://reviews.apache.org/r/11473
Added 'ThreadLocal' to stout/thread.hpp.

Review: https://reviews.apache.org/r/11474
Used ThreadLocal from stout.

Review: https://reviews.apache.org/r/11475
Fixes os::environ for OS X.