It was love at first knee to the groin. Nikki Baker, 22, and John Dahl, 25, met in high school; he got her attention by shaking a can of beer in front of her, then cracking it open so a geyser of foam blasted into her face. She returned his affection with a brutal blow to the crotch. Today, their finely honed Honeymooners-meets-Jackass revenge routine is a runaway hii