Entity and Block registration now use integers instead of strings

And a couple of tweaks here and there
Minor fixes
Implemented full mod system. Lacking some events maybe, but loading is fully functional.

To do that, I added a new dependency: Reflections, and also added a couple Reflection-related util methods

Mods' spec:

- Must have @Mod annotation
- Make methods with @Subscribe, and the single argument either ModLoadEvent or ModUnloadEvent (or even ModEvent) to be called when those are posted (method name does not matter)