Refactor: easybib_crontab

 * moved cron parsing logic into a library
 * exit early from provider code to avoid large if/else

 * todo:
   * test (this is just a WIP)
   * discuss this approach
   * add a test to confirm parsing works as expected
Fix: please rubocop
Enhancement: add a simple unit test for EasyBib::Cron
Fix: rubocop offense
Update: move library
Fix: try to handle missing/non-existing crontab more gracefully

Fixes the following error in a chef-run:
DEBUG: LocalJumpError: easybib_crontab[gearmanworker] had an error: LocalJumpError: unexpected return
Fix: another attempt fixing the JumpError

Return with next, because Ruby.
Enhancement: WIP, test to confirm easybib_crontab refactorings don't break when no crontab is present

cc @fh
Fix the test for nonexisting crontab
Merge pull request #287 from till/refactor/easybib_crontab

Refactor: easybib_crontab