examples: added examples/event/damage,hostile,melee.rb; examples are for wip new api
wip on examples/event/damage, melee
EventTarget: add #observers, replace #each_bind with #each_matching_callback, move EventBind and EventBindDisconnector to new files (renamed BindDisconnector -> EventBindDisconnector)
Event: update to use EventTarget#each_matching_callback and rewrite collect_targets algorithm