Add separate KB textures for each faction

-- Alliance uses existing texture (renamed to KillingBlow_Alliance.tga);
Horde uses the new KillingBlow_Horde.tga (by OligoFriends)
-- Move the texture:SetTexture call to the first firing of
PLAYER_ENTERING_WORLD so UnitFactionGroup can return accurate results
- Rename frame to KillingBlow_EnhancedFrame (was KillingBlowImageFrame,
from the AddOn's old name)