Bukkit

+ Added more annotations for null safety (developer stuff)

+ Added java doc comments (developer stuff) (don't know if anyone is interested in an online java doc, let me know if you are)

* Fixed issue #70 (GitHub): Signs error

* Fixed issue #69 (GitHub): Critical issue with arenas

* Fixed issue #68 (GitHub): exception error while execution task xxx and problem with voting

* Fixed issue #67 (GitHub): Cannot vote once your own plot has been voted on

* Schedule main scoreboard on join a tick later

* Changed Minecraft version to 1.12, 1.11 is no longer supported