Improve Deathrattle triggering

Instead of having buffs trigger their own deathrattles, we now loop
over self.deathrattles which gives a list of all the deathrattles to
trigger.

This means that hasDeathrattle needs to become a property so that
minions without deathrattles can find their buffs' deathrattles.

Isn't Soul of the Forest fun?
Allow for virtual auras

Some auras are not defined in the game card. This change lets us
define auras as "class Aura" from within the card with the Aura.
Implement Baron Rivendare, with tests