Cleanup and fixes
- General cleanup of unused imports
- JsonConfigHandler added null checks
- SkillStore added single BlockSkill add Method to be consistent with PerkStore

- BlockPerk moved default constructor (Parameterless) to the top
- EntityPerk moved default constructor to the top

 - BlockSkill refactored the default constructor to instantiate a new ArrayList<BlockXP> and set this as the base exp

 - EntitySkill refactored the default constructor to instantiate a new ArrayList<EntityXP> and set this as the base exp

 - ItemSkill refactored the default constructor to instantiate a new ArrayList<ItemXP> and set this as the base exp

 - Skill refactored the ArrayList to have generic parameter
 - Skill refactored method getExp() to return a ArrayList<? extends XPGain>

 - BlockBreaking and EntityDamage, added casts where getting the exp from the relevant subclasses of skills

 - .gitignore added server and client files to the ignore list