Support more NBT files

- NBT files can now be uncompressed
- NBT files can now have the tags directly in the root (doesn't need to
be wrapped in a TagCompound anymore)

- Add a notice to /tc list on how to get the world objects
Fix support for some NBT files, fix CustomStructure

- NBT files where all tags are directly in the root (as opposed to
wrapped in a Compound Tag) that have nested id tags (for item ids for
example) are now supported.
- CustomStructure can now accept more than one structure
- CustomStructure now uses doubles for the rarity.
- Added @Overrides to BukkitBiome.
Update version to 2.4.7
Remove some unused imports