Extra powerup and hints

Another small tweak!

I have added a shield powerup - collect this to temporarily become invulnerable to dogs crashing into you.  Duration of the powerup depends on difficulty level (same as the star powerup).

Also added a hint when speed increases (no speed increase in very easy mode; 2 speed increases in other modes, amount of dogs fed prior to increases depends on difficulty.)

If you have any comments on the changes, or ideas for improvements, please let me know.  :)