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/core/apis/FSAPI.java

@@ -43,9 +43,7 @@ public class FSAPI implements ILuaAPI
     @Override
     public String[] getNames()
     {
-        return new String[] {
-            "fs"
-        };
+        return new String[] { "fs" };
     }
 
     @Override