Taken from https://github.com/davidmoreno/garlic
See also http://blog.coralbits.com/2014/03/c-cron-class-implementation.html

License: "The code is MPL 2.0, so use it in your own projects, but future version may change that to Apache2."

Changes:
* Added #include <unistd.h> to cron.cpp because sleep requires it.
