guards can become alerted, and stop moving when they see you.
bug: fixed a weird issue when walking into a guard who notices you sets hero back to 0,0.  This must have something to do with javascript.
   I called guards[i].target = hero; rather than guards[i].target = {x: hero.x, y: hero.y}; I think target became a pointer for hero and then
   target.x and .y got set to null or 0 or something.
Guards rotate to follow hero with sight.
Sprites rotate to face target.
bug (unsolved): guards can see you through walls, fix with raycasting
add alert_guards.png