Add a little bit of source code checking to Gradle

- Adds a CheckStyle configuration which is pretty similar to CC's
   existing one.
 - Add the Gradle license plugin.
 - Ensure the existing source code is compatible with these additional
   checks.

See #239

src/test/java/dan200/computercraft/shared/wired/NetworkTest.java

@@ -253,14 +253,14 @@ public class NetworkTest
         assertEquals( Sets.newHashSet(), cE.allPeripherals().keySet(), "C's peripheral set should be empty" );
     }
 
+    private static final int BRUTE_SIZE = 16;
+    private static final int TOGGLE_CONNECTION_TIMES = 5;
+    private static final int TOGGLE_NODE_TIMES = 5;
+
     @Test
     @Disabled( "Takes a long time to run, mostly for stress testing" )
     public void testLarge()
     {
-        final int BRUTE_SIZE = 16;
-        final int TOGGLE_CONNECTION_TIMES = 5;
-        final int TOGGLE_NODE_TIMES = 5;
-
         Grid<IWiredNode> grid = new Grid<>( BRUTE_SIZE );
         grid.map( ( existing, pos ) -> new NetworkElement( null, null, "n_" + pos ).getNode() );
 
@@ -324,7 +324,7 @@ public class NetworkTest
         }
     }
 
-    private static class NetworkElement implements IWiredElement
+    private static final class NetworkElement implements IWiredElement
     {
         private final World world;
         private final Vec3d position;
@@ -433,7 +433,7 @@ public class NetworkTest
         private final T[] box;
 
         @SuppressWarnings( "unchecked" )
-        public Grid( int size )
+        Grid( int size )
         {
             this.size = size;
             this.box = (T[]) new Object[size * size * size];