Refactor code so that we support more CRON definitions (Unix, Quartz, ...)

... and provide means to support more definitions in future.
Currently we support Unix (http://unixhelp.ed.ac.uk/CGI/man-cgi?crontab+5) and Quartz (http://quartz-scheduler.org/documentation/quartz-2.x/tutorials/crontrigger) formats.