Added new functions to World.java

Hi,

I added new functions to the World.Java interface, I used the level.dat NBT file as a reference, and added a function for almost every tag in the file that might be useful in the future (when working on world generation/loading).

Functions I added:
* getGamerule
* getTime
* getSpawn
* isRaining
* getRainTime
* isThundering
* getThunderTime
* canGenerateStructures
* getBorderSize
* getBorderCenter
* getBorderSizeContraction
* getBorderSizeContractionTime