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/AbstractTurtleUpgrade.java

@@ -1,9 +1,8 @@
 /*
- * This file is part of ComputerCraft - http://www.computercraft.info
- * Copyright Daniel Ratcliffe, 2011-2019. Do not distribute without permission.
- * Send enquiries to dratcliffe@gmail.com
+ * This file is part of the public ComputerCraft API - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2019. This API may be redistributed unmodified and in full only.
+ * For help using the API, and posting your mods, visit the forums at computercraft.info.
  */
-
 package dan200.computercraft.api;
 
 import dan200.computercraft.api.turtle.ITurtleUpgrade;