Crontab class no longer requires params

As the init.pp includes the crontab class automatically rather than explictly it
may be the case that the user does not pass any parameters for the $jobs
parameter to the class. This commit adds logic to only create jobs if a hash if
passed, otherwise if anything but undef is recived an error is produced.
Merge pull request #5 from george-b/emptpy_params

Crontab class no longer requires params