API: KeyValue storage

- [x] Code
- [x] Test
- [ ] Documetation

### Reference

- [Drupal::keyValue](http://goo.gl/eI64uX)
- [namespace Drupal\Core\KeyValueStore](http://goo.gl/9xsz5x)
- [KeyValueExpirableFactoryInterface](http://goo.gl/lQ96pm)
- [StateInterface](http://goo.gl/dVM6MM)
- [KeyValueStoreInterface](http://goo.gl/kNfvlr)