Add support for delegating permissions to Sponge.

When Sponge is not installed, behaviour is as before.
Closes #331

config/checkstyle/import-control.xml

@@ -55,6 +55,7 @@
       <allow pkg="org.apache.logging.log4j"/>
       <allow pkg="org.lwjgl"/>
       <allow pkg="io.netty.buffer"/>
+      <allow pkg="org.spongepowered.api" />
     </subpackage>
   </subpackage>
 </import-control>