Added an HTML index page so that the whole lib can be played with in a browser console
Refactored the Skill (now CharacterSkill) object and factory methods

The initialization has been moved to a dedicated method, and the factory method put back into the CharacterSkill object itself (instead of the BattleAxe global object).

The properties of a skill object have also been defined, so that immutable properties are actually immutable, while mutable properties are properly mutable.