Difficulty adjustments & some visual things

In addition to difficulty level affecting starting lives & amounts of food, it now changes the delay before the first dog spawn and the spawn interval.  Also affected are the in-game visual aids - easy difficulty includes icons above the dogs to tell you which food to use, and now hard difficulty does not include an aim line.  The difficulty buttons now have tooltips to help you decide which level to try.  The story/instruction screen is now a little prettier too.