Change how target tracking is handled to be more generic/consistent.

Core

• Add Target class with generic functions
• Reorganize source files
• Refactor TimeToDie

Demon Hunter

• Havoc

Druid

• Balance
• Feral
• Restoration

Evoker

• Devastation
• Preservation
• Augmentation

Hunter

• Beast Mastery
• Marksmanship
• Survival

Monk

• Mistweaver
• Windwalker

Priest

• Holy
• Shadow

Rogue

• Assassination
• Outlaw

Shaman

• Elemental
• Enhancement
• Restoration

Warrior

• Arms
• Fury