Implemented rising difficulty  + Changelog

Changelog:
@ Ball.java
- removed javax.swing.JOptionPane;
- "int pointscounter" to "public static int pointscounter"
- !!!ADDED inceaseDiff Method!!! ( reduces racquet and ball by 10% each
15 hits, and increasess ball speed by 20%  each 30 hits.)
- Static access to Color.

@ Game.java
- Increased Raqcuet Lenght to 120.
-Added gameSpeed variable.
-removed change difficulty block
- Thread.sleep(); now intialises with gameSpeed

@ Racquet.java
- racquetLenght set from private to public fo access.