Support different NMS block classes depending on version.
- The .class files in the contrib folder of the zip go in
  plugins/WorldEdit/nmsblocks
- This allows us to swap new class files in without releasing
a completely new version each time
- Whatever version the last release is for has an inbuilt fallback
- If the plugin and server are mismatched and you have nothing in
  nmsblocks you're screwed
WEPIF GroupManagerResolver implementation