Additional refactoring of the effects system.

The system formerly used only for Buff was refactored into a
superclass called EventEffect that can be used by any effect that
responds to particular events happening to a minion.

Kill on Freeze on damage was moved to the EventEffect system.
Updated readme.md to point to hs-ai.com
Created EventEffectPlayer for effects that affect the minon's player.

These are typically to do with drawing cards.  Also fixed some bugs with
KelThuzad and made a few more minions able to be copied.
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.
Merge branch 'master' into serialization

Conflicts:
	hearthbreaker/effects/minion.py
	hearthbreaker/game_objects.py
	hearthbreaker/replay.py
Added __to_json__ to all minion effects
Implemented __from_json__ for minion effects

Most minions that are copyable are now serializable as well.