Implemented support for empty entry. Fixed one case with empty or whitespace only crontab entry causing code to crash.

Signed-off-by: Daniel (dmilith) Dettlaff <dmilith@verknowsys.com>
Added more tests covering fixed empty cron entry case.

Signed-off-by: Daniel (dmilith) Dettlaff <dmilith@verknowsys.com>
Updated README.

Signed-off-by: Daniel (dmilith) Dettlaff <dmilith@verknowsys.com>