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

Core

• Add Debuff class with generic methods
• Reorganize source files

Druid

• Balance
• Feral
• Restoration

Hunter

• Beast Mastery
• Marksmanship
• Survival

Monk

• Windwalker

Priest

• Holy
• Shadow

Rogue

• Assassination
• Outlaw

Shaman

• Elemental
• Enhancement
• Restoration

Warrior

• Arms