Add CronjobShell

Allow manipulation of the crontab daemon from within a cake shell


```
www-data @ web [ /home/cw/www/platform/htdocs/app ] (develop *%=)
-> Console/cake Common.cronjob 
Usage:
cake common.cronjob [subcommand] [-h] [-v] [-q]

Subcommands:

lint    Validate syntax and basic data requirements for your
        cronjobs.json file
show    Convert cronjobs.json to a crontab compatible output. This will
        also lint the file to ensure correct syntax
export  Export all cronjobs.json cronjobs into crontab. This command
        will also remove project cronjobs that no longer exist in cronjobs.json
clear   Remove all cronjobs belonging to the current project from
        crontab

To see help on a subcommand use `cake common.cronjob [subcommand] --help`

Options:

--help, -h     Display this help.
--verbose, -v  Enable verbose output.
--quiet, -q    Enable quiet output.
```