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/main/java/dan200/computercraft/api/ComputerCraftAPI.java

@@ -438,45 +438,45 @@ public final class ComputerCraftAPI
                 computerCraft_getVersion = findCCMethod( "getVersion", new Class<?>[] {
                 } );
                 computerCraft_createUniqueNumberedSaveDir = findCCMethod( "createUniqueNumberedSaveDir", new Class<?>[] {
-                    World.class, String.class
+                    World.class, String.class,
                 } );
                 computerCraft_createSaveDirMount = findCCMethod( "createSaveDirMount", new Class<?>[] {
-                    World.class, String.class, Long.TYPE
+                    World.class, String.class, Long.TYPE,
                 } );
                 computerCraft_createResourceMount = findCCMethod( "createResourceMount", new Class<?>[] {
-                    Class.class, String.class, String.class
+                    Class.class, String.class, String.class,
                 } );
                 computerCraft_registerPeripheralProvider = findCCMethod( "registerPeripheralProvider", new Class<?>[] {
-                    IPeripheralProvider.class
+                    IPeripheralProvider.class,
                 } );
                 computerCraft_registerTurtleUpgrade = findCCMethod( "registerTurtleUpgrade", new Class<?>[] {
-                    ITurtleUpgrade.class
+                    ITurtleUpgrade.class,
                 } );
                 computerCraft_registerBundledRedstoneProvider = findCCMethod( "registerBundledRedstoneProvider", new Class<?>[] {
-                    IBundledRedstoneProvider.class
+                    IBundledRedstoneProvider.class,
                 } );
                 computerCraft_getDefaultBundledRedstoneOutput = findCCMethod( "getDefaultBundledRedstoneOutput", new Class<?>[] {
-                    World.class, BlockPos.class, EnumFacing.class
+                    World.class, BlockPos.class, EnumFacing.class,
                 } );
                 computerCraft_registerMediaProvider = findCCMethod( "registerMediaProvider", new Class<?>[] {
-                    IMediaProvider.class
+                    IMediaProvider.class,
                 } );
                 computerCraft_registerPermissionProvider = findCCMethod( "registerPermissionProvider", new Class<?>[] {
-                    ITurtlePermissionProvider.class
+                    ITurtlePermissionProvider.class,
                 } );
                 computerCraft_registerPocketUpgrade = findCCMethod( "registerPocketUpgrade", new Class<?>[] {
-                    IPocketUpgrade.class
+                    IPocketUpgrade.class,
                 } );
                 computerCraft_getWirelessNetwork = findCCMethod( "getWirelessNetwork", new Class<?>[] {
                 } );
                 computerCraft_registerAPIFactory = findCCMethod( "registerAPIFactory", new Class<?>[] {
-                    ILuaAPIFactory.class
+                    ILuaAPIFactory.class,
                 } );
                 computerCraft_createWiredNodeForElement = findCCMethod( "createWiredNodeForElement", new Class<?>[] {
-                    IWiredElement.class
+                    IWiredElement.class,
                 } );
                 computerCraft_getWiredElementAt = findCCMethod( "getWiredElementAt", new Class<?>[] {
-                    IBlockAccess.class, BlockPos.class, EnumFacing.class
+                    IBlockAccess.class, BlockPos.class, EnumFacing.class,
                 } );
             }
             catch( Exception e )