Refactor code to enable handling different CRON string definitions (Unix, Quartz, ...)

Hello, 

I made a small refactor on code, so that is now able to handle different CRON definitions. Now it can handle Unix cron as well as Quartz (http://quartz-scheduler.org/documentation/quartz-2.x/tutorials/tutorial-lesson-06) and provides means to extend to new cron definitions.