Added library to shutdown / reboot

Added library to shutdown / reboot computer right now or in the future. 

Examples:

* betty shutdown computer now
* betty shutdown computer in 30 min
* betty reboot system in 2h
* betty restart computer at 23:59

Note : That pull request is the same that : https://github.com/pickhardt/betty/pull/143 with a cleaner commit history...
Nice work, just add one for cancel and I'll accept it.