Add Unit 'slice'

[icanhazstring]

First of all, the Slice unit type should implement the UnitInterface similar to the Timer or Service.
Despite that everything, that is possible with a Timer should also be possible with a Slice.

In the most simple case, you only need to provide the Slice class implementing the UnitInterface extending AbstractUnit. The class should provide the correct suffix.

For more information about a Unit or Slice see systemd documentation:
https://www.freedesktop.org/software/systemd/man/systemd.unit.html
https://www.freedesktop.org/software/systemd/man/systemd.slice.html

Also see PR #27 from @peter279k for reference.