Fixed signal handling for stout.

Review: https://reviews.apache.org/r/25126
Fixed signal handling in libprocess.

Review: https://reviews.apache.org/r/25128
Fixed signal handling for Mesos.

Review: https://reviews.apache.org/r/25129
Improved the signal handling comment in Mesos tests.
Fix External Containerizer creation.

Review: https://reviews.apache.org/r/25116
Modifed execute cli to run docker images.

Review: https://reviews.apache.org/r/24808
require some c++11 features for libprocess

Review: https://reviews.apache.org/r/25168
require some c++11 features for mesos

Review: https://reviews.apache.org/r/25169
Changed the stout path utility to declare a single, variadic 'join' function instead of several separate declarations of various discrete arities.
Fixes for proper Mesos Style
* Missing space around colon in range-based for loop
* Redundant blank line at the end of a code block should be deleted.
* Namespace should be terminated with "// namespace path {"
remove whitespace
Added unit a unit test for stout/path, but it isn't running. Also closed some issues to do with forming absolute paths.
Added path_tests to Makefile in libprocess/3rdparty. Test now runs during make check. It also passes.
Guard C++11 parts with ifdef.
Flipping ifdef logic, and addressing style issues.
Added Apache license block.