Improved Javadoc (use @param, @return, etc).

Review: https://reviews.apache.org/r/19859
Added os::environment.

Review: https://reviews.apache.org/r/19948
Moved subprocess and cleanup method to new cpp.

Review: https://reviews.apache.org/r/19416
Added optional environment map to subprocess.

Review: https://reviews.apache.org/r/19162
Adjusted the includes in stout/net.hpp to avoid definition conflicts caused by netlink.
New and improved stout README.