Add method to prevent a null return of World#loadChunk

Added a method as suggested by @sk89q that automatically generates the chunk if it is loaded and does not exist rather than only generating if shouldGenerate is true. If should generate is false the method would normally return null. This method should never return null. The missing Nullable annotations are added in a seperate PR shown here: https://github.com/SpongePowered/SpongeAPI/pull/155. This also fixes a markdown tick
The point here is shouldGenerate is true, so this will never return null.