Uniform method signature
Change how to test generated commands
Change how to test cron type

* Remove shared_cron_examples for easy understanding by loosing abstraction
* Run matcher test and command test at same time

I will change tests for other types like this.
Remove stand-alone commands tests for cron

They were integrated to matcher tests.
Merge pull request #177 from serverspec/fix-cron-tests

Change how to test cron type