Began re-working auras to work with more than just attack/health
Completed AuraEffect and its needed subclasses
Moved all aura creating minions to the effect system.

Debugged the application of auras and moved it to all the methods that
actually affect board state (add_to_board, silence, replace, remove_from_board)

It's possible that this could be streamlined, but I'm not sure how just yet.