Added `DismissibleEvent` and `MessageEvent`

* each response will now include a `display` option (which is assumed to be 'button' if not present)
* if `display` is 'background', the client should respond when the message background is tapped
* `ResourceHarvestEvent` and `MessageEvent` now inherit from `DismissibleEvent`
Merge branch 'day_stage' of https://github.com/iteloo/the_island into day_stage