Refactor Yadc::Core::Skills::Move skills

* Add specs
Improve Yadc::Core::Dungeon::Level specs
Remove spec_helper requirements
Replace “be_truthy” and “be_falsey” by “be true” and “be false”
Replace “describe ‘initialization’” by “describe ‘#initialize’”
Rename Yadc::Core::Skills module to Yadc::Core::Skill
Rename Yadc::Core::Skill::Move to Yadc::Core::Skill::Walk
Clean up file requirements