made final

src/main/java/com/xeiam/xchart/BitmapEncoder.java

@@ -35,7 +35,7 @@ import javax.imageio.stream.FileImageOutputStream;
  * 
  * @author timmolter
  */
-public class BitmapEncoder {
+public final class BitmapEncoder {
 
   /**
    * Constructor - Private constructor to prevent instantiation

src/main/java/com/xeiam/xchart/ServletEncoder.java

@@ -27,7 +27,7 @@ import javax.servlet.ServletOutputStream;
  * 
  * @author timmolter
  */
-public class ServletEncoder {
+public final class ServletEncoder {
 
   /**
    * Constructor - Private constructor to prevent instantiation