Improved event handling and stage handling; fixes #13

* stage/events now end by adding to ioloop (hosted by `server`) callback to their delegates (game/player)
* event_queue --> _event_queue is now a hidden var of `Player`
* schedule new events using `schedule_event(s)` on `Player
Merge branch 'day_stage' of https://github.com/iteloo/the_island into day_stage
Added `EventHandler` with stack feature

* moved event handling from `Player` to `EventHandler`
* event handler is either processing or not
* when it is processing, it will push new events from the queue (if any) whenever the stack is empty
* when it is not processing, it does nothing except wait for the stack to deplete
Merge branch 'day_stage' of https://github.com/iteloo/the_island into day_stage
Implemented `item_activated`

* Also, created setter methods for player inventory and condition