Entities emit "destroyed" events.

Signed-off-by: suicidepills <atom.mac@gmail.com>
Make ShockNova an Entity.

Signed-off-by: suicidepills <atom.mac@gmail.com>
Don't update() or draw() ShockNova from Character.

Since ShockNova is now an Entity, PlayState will call update() and draw()
on it during the game loop.

Signed-off-by: suicidepills <atom.mac@gmail.com>
Listen to "destroyed" events from PlayState.

Signed-off-by: suicidepills <atom.mac@gmail.com>
Merge branch 'master' of https://github.com/suicidepills/hack-n-slash-sprint