Add Weather API

Added weather listeners. Also I decided to add the api.weather package and the Weather/WeatherType files so that you can easily access the current weather from World with something like getWeather() that would return a Weather object that could be manipulated. This in unlike Bukkit where you had the hasStorm() and isThundering() in the World methods but no direct way to get weather. 

Any ideas about this? Would it be nice to have a world.getWeather ability? If so where would that be? Extent? World? etc..?
Optional<Entity> never null.