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

src/main/assembly/default.xml

@@ -28,6 +28,7 @@
         <include>LICENSE.txt</include>
         <include>CHANGELOG.txt</include>
         <include>contrib/craftscripts/*</include>
+        <include>contrib/nmsblocks/*.class</include>
       </includes>
     </fileSet>
   </fileSets>