Alchemical Arrows 3

* Fixed a NullPointerException when regular arrows hit the ground
* Switched the update checker from using BukGet to SpiGet. (Rest in peace, BukGet project :c)
* Now compiled with Java 8 rather than Java 7
* Skeletons now have a 25% chance to drop alchemical arrows (configuration options added per arrow)
* Rewrote a little bit of the internal workings of the API to ease development
* Renamed all listener classes to include "Listener" at the end
* API: Added an AlchemicalArrow#skeletonLootWeight() method, obviously, indicating the chance that the arrow will get dropped
* API: Changed the arrow registry Map to a BiMap
* API: Added an ArrowRegistry#getInformationalInstance() method to retrieve data from arrows
* API: Methods in the arrow registry that return Maps / Sets are now immutable