Extended World API

Adds a lot of methods to the world and chunks as discussed here: https://github.com/SpongePowered/SpongeAPI/issues/256

**Small Overview:**
- `Server`
  - `createOrLoadWorld(...)`
- `Events`
  - `ChunkPreGenerate/Populate/DecorateEvent`
  - `ChunkPostGenerate/Populate/DecorateEvent`
- `StorageContainer`
- `World`
  - `getStorage()`
  - `getExistingRegions()`
  - `getExistingChunks(...)`
  - `getChunkData(...)`
  - `deleteChunk(...)`
- `Region`
  - `getStorage()`
  - `getExistingChunks(...)`
- `Chunk`
  - `getWorld()`
  - `getChunkData()`
- `ChunkData`
  - `get...Position(...)`
  - `getChunkState()`
- `ChunkState`
  - `NOTAVAILABLE`, `GENERATED`, `POPULATED`, `DECORATED`, `LOADED`
- `ChunkGenerator/Populator/Decorator`

RFC
Ok i will remove the Optional.

In the last part you got me wrong. Plugins will not throw or call it. They will listen to the event to store some metadata. And the event will always be thrown called regardless of the engine specifing a decorator or not. This is now obsolete.

If there is always a polulator and decorator would it make sense to change them to a list so plugins are able to add a decorator to the world instead of just doing all the stuff in the event? I dont want to remove the event just add another way of doing it.