Improve existing code

- [ ] Refactor classes
- [ ] Add missing specs
- [ ] Fix existing specs
- [ ] Use bang methods where appropriate
- [ ] rename `Skills` to `Skill`
- [ ] rename `Skill::Move` to `Skill::Walk``
- [ ] add a `Skill::Turn`
- [ ] skills return true if executed, false otherwise
- [ ] rewrite doc with YARD